APPENDIX 1

The Bacteria

Identification Scheme for Selected Gram-Positive Bacteria	A1-2
Identification Scheme for Selected Gram-Negative Bacteria	A1-3
Alphabetic listing of organisms	A1-4

Acinetobacter: Organisms in this gram-negative genus are found in water, food, soil, and are recoverable from most areas of the human body. They may cause infections, particularly in debilitated persons.

A. anitratus is occasionally mistaken for Neisseria species in smears of spinal fluid and pus. Having been confused for gonococci, this organism has prompted erroneous reports of penicillin-resistant cases of gonorrhea. It is part of the normal flora of the vagina, skin, respiratory, tract, and GI tract but may produce conjunctivitis, meningitis, SBE, urinary tract infections and vaginitis, and may contaminate inhalation and dialysis equipment.

A. lwoffi is part of the normal flora of the skin, mouth, and genitalia. It may cause septicemia, SBE, and pneumonia in debilitated persons.

Aeromonas and Plesiomonas: Organisms of these gram-negative genera are found in water, soil, and occasionally the stools of asymptomatic individuals. They produce diseases primarily in fish and amphibians.

P. shigelloides has been associated with human diarrhea.

A. hydrophilia has been found in cases of wound infections, urinary tract infections, and diarrhea.

Alcaligenes: Members of this gram-negative genus are part of the normal intestinal flora but differ from the Enterobacteriaceae in being oxidase positive and in failing to ferment or oxidize any of the usual carbohydrates. Its energy is derived from the oxidation of organic acids and amino acids. This makes the medium alkaline and leads to confusion with the non-lactose fermenters such as Shigella and Salmonella.

A. faecalis has occasionally been isolated from infants with septicemia and meningitis. It may contaminate intravenous solutions and cause post-operative septicemia.

Anaerobic cocci: These include Peptostreptococcus, Veillonella, and Peptococcus niger which are part of the normal flora, particularly the mouth, upper respiratory tract, and large intestine. They are found in a variety of infections, e.g., abcesses of soft tissue, surgical wounds, respiratory tract, female genital tract, septic arthritis, neck, and dento-alveolar tissue.

Arizona: This Enterobacteriaceae genus resembles Salmonella and Citrobacter, but unlike them, slowly liquifies gelatin. In man it may produce gastroenteritis, urinary tract infection, bacteremia, and meningitis. A. hinshawii is the only species.

Bacillus cereus. B. cereus is a large (1 x 3-5 mm), motile, gram-positive, spore-forming bacillus found in soil. It is a cause of food-poisoning. The heat-resistant endospores can survive inadequate cooking, and if the food is then held at a growth-permissive temperature for long enough, germination will occur, accompanied by production of enterotoxins. B. cereus food poisoning can have two clinical presentations, an emetic illness with a short incubation period (1-6 h), that resembles gastroenteritis due to S. aureus, and a diarrheal syndrome with a longer incubation period (6-24 h), that resembles food poisoning due to Clostridium perfringens. The two presentations are caused by two different enterotoxins. In this country, fried rice is the major food implicated in the emetic type of B. cereus food poisoning. The bacteria are commonly present on uncooked rice. The toxin that causes the emetic illness is heat-stable and will survive rapid cooking methods such as stir-frying. Proof of B. cereus food poisoning requires isolation of B. cereus from patient stools; the diagnosis can be confirmed by isolating > 10⁵ organisms per gram of the implicated food.

Bacteroides, prevotella, and porphyromonas: The members of these genera are gram-negative, nonsporeforming anaerobes with varying degrees of sensitivity to oxygen.

B. fragilis and B. thetaiotaomicron are the most clinically significant Bacteroides. They are part of the normal flora of humans and animals, found principally in the intestinal tract, occassionally in the female genital tract. They are most often isolated from intraabdominal infections, but may be seen elsewhere.

Porphyromonas gingivalis, Prevotella loescheii, and Prevotella melaninogenicus are closely related to the Bacteroides. These two genera contain several species, but the most commonly isolated are these three. The first two of these are normal flora in the human mouth, and are often isolated from oral and dental infections. They may also be found in head, neck, and lower respiratory tract infections. Other related species are also normally found in the mouth, but can also be found in the urogenital and intestinal tracts. They can be found in variety of infected sites, particularly head, neck, oral, dental, bite, and pleuro-pulmonary infections.

Bordetella pertussis. B. pertussis causes pertussis (whooping cough). Humans are the only host infected by B. pertussis; the gram-negative organism is spread aerogenically by infected infants and young children. It probably also is transmitted by adults, who get only a mild disease. B. pertussis is isolated from the posterior nares and respiratory tract of children with whooping cough only during the first several weeks of their illness. Unlike most other potential pathogens residing in the pharynx, B. pertussis almost never produces a bacteremia. The organisms are tiny nonmotile Gram-negative coccobacilli and are obligately aerobic. They set up infections only in respiratory tissue. B. pertussis is isolated most readily on Bordet-Gengou agar plates, which contain 15% sheep blood (vs. the 5% in an ordinary BAP) and potato starch (for binding fatty acids in the blood which are toxic to this fastidious organism). It is identified most rapidly by immunofluorescence staining.

Campylobacter: Obligately microaerophilic, gram-negative curved rods. Most spp. cause gastroenteritis in humans.

C. jejuni is the most often implicated species in the USA.

C. fetus causes a slightly different type of enteric disease, in which following infection of the small bowel the organism can become systemic and cause a typhoid-like disease.

Chromobacterium: A pleomorphic gram-negative rod whose colonies form violet pigments. Commonly found in the soil and water.

C. violaceum has rarely produced infantile diarrhea with a high mortality rate.

Citrobacter: These Enterobacteriaceae organisms have been associated with enteritis, urinary tract infections, pneumonia, and burn infections. A few strains resemble Salmonella in being lac-negative and in their similar IMViC reactions (-/+/-/+). However, unlike Salmonella, Citrobacter species do not decarboxylate lysine.

C. freundii and C. diversus are the commonly encountered species.

Clostridium: This gram-positive genus of sporeforming anaerobic bacilli include several important human pathogens. These bacteria are widely distributed in soil and water around the world, but are sometimes part of the normal flora of (some) humans intestines.

C. difficile causes pseudomembranous colitis and antibiotic-associated diarrhea.

C. perfringens is found in infections in many parts of the body. Common infections include gas gangrene, wound contamination, postabortion infection, necrotizing pneumonia, and empyema. It also commonly causes food poisoning, generally being the third most frequent cause of food-borne gastroenteritis in the United States. Improperly cooked (or stored) meat is the usual food vehicle. C. perfringens also causes a severe illness termed enteritis necroticans.

C. tetani causes tetanus. It is a disease of the unimmunized in the United States. It is not difficult to diagnose.

C. botulinum causes four types of disease: 1) food-borne botulism, an intoxication; 2) wound botulism, a rare disease caused by direct infection of a wound; 3) infant botulism, caused by botulinum toxin produced by C. botulinum that have colonized the infant's intestines; 4) an infection of children and adults in which food and wounds are not the source, but the actual source is unknown. Infant botulism is the most common of these.

Coliform: This is a general term for fermentative gram-negative bacilli which normally reside in the GI tract of people without causing disease. To some, the term covers all enteric bacilli, while to others it includes only the lac-positive enteric organisms. However, for most the coliform group now encompasses E. coli, Citrobacter, Klebsiella, Enterobacter, Serratia, Morganella, Proteus, and Providencia.

Corynebacterium diphtheriae. C. diphtheriae is a gram-positive, nonmotile rod that is facultatively anaerobic but grows best aerobically. It infects the upper respiratory tract and skin, causing the disease called diphtheria. Its major established virulence property is diphtheria toxin. This toxin is an exotoxin that accounts for most of the pathologic effects of diphtheria: it causes both local and systemic symptoms. Diphtheria toxin is produced only by strains of C. diphtheriae that are lysogenized by a temperate bacteriophage, corynephage b, carrying the structural gene for toxin. There are many serotypes of C. diphtheriae, but all make an identical toxin. Humans are the main reservoir (the bacteria are carried in the respiratory tract). Infection is spread by droplet transmission or contact with nasal secretions.

In diphtheria, the bacteria spread from the nasopharynx to the larynx and trachea and cause severe disease. In addition to serious systemic effects of the toxin (such as cardiac arrest or respiratory paralysis), diphtheria patients can suffer asphyxiation due to a pseudomembrane that forms at the site of the infection. The airway can be obstructed by this and by the local swelling. Although diphtheria is primarily a disease of the upper respiratory tract, primary or secondary lesions can occur in other parts of the body. Skin is the most common of these (due to infection of an abrasion) and results in chronic, spreading ulcers covered by a greyish membrane. This is more of a problem in the tropics than in the temperate zone (however, physicians here will see immigrants with skin diphtheria, and this infection is increasingly seen among street people in the US). People also can be infected by a nontoxigenic strain of C. diphtheriae, with a resulting mild "surface" diphtheria due to effects of virulence factors such as cord factor. "Diphtheroids" are members of our normal flora that are morphologically similar to diphtheria bacilli -- e.g., C. pseudodiphtheriticum and C. xerosis. These are relatively nonpathogenic; they are opportunistic pathogens.

Typically, in cases of suspected diphtheria, the physician will provide the diagnostic lab with nose and throat swabs, which are used to streak a tellurite plate (selective for corynebacteria), a BAP, and a Loeffler slant. This last is a suboptimal medium that accentuates some diagnostically useful morphological characteristics of C. diphtheriae, such as pleomorphism and the presence of metachromatic granules of polyphosphate called Babes-Ernst granules. C. diphtheriae stains poorly with the Gram stain, and other stains, such as methylene blue, are useful. Suspected isolates of C. diphtheriae are tested for toxigenicity. One such test is an antigen-antibody test called the Elek plate. If the test organism produces diphtheria toxin, a white line of toxin-antitoxin precipitate forms at a 45^o angle from the intersection of antitoxin-impregnated filter paper and the culture streak.

Edwardsiella: This Enterobacteriaceae genus resembles Salmonella in being lac-negative, motile, and H₂S-producing. Unlike Salmonella, these organisms are indole positive. They have been isolated from many mammals and in people appear to be only an accidental invader, producing, rare, acute gastroenteritis. E. tarda is the only species.

Enterics: This term includes all bacteria normally encountered in the human gastrointestinal tract.

Enterobacter: This Enterobacteriaceae genus is present in soil, water, dairy products, and the GI tract. In some hospitals, it is the third most common cause of bacteremia in man. Most Enterobacter infections are associated with a depressed host resistance.

E. aerogenes is regularly present on vegetation and, in general, is a harmless contaminant of drinking water, milk, etc. Its presence here is of little consequence in contrast to that of E. coli, which implies fecal contamination and raises the possibility of Salmonella and other intestinal pathogens also being present. Many of the differential media in use today were devised initially by public health people seeking a means of distinguishing quickly between E. coli and E. aerogenes. IMViC is a set of tests still used today for this purpose.

E. cloacae, E. aerogenes, and E. agglomerans are the species most commonly encountered in people.

Enterobacteriaceae: These gram-negative non-spore-forming rods grow readily on simple media under aerobic and anaerobic conditions. They ferment glucose with the formation of acid or of acid and gas. They give a negative oxidase reaction. Intra-family differentiation is based on

a) their motility: Shigella and Klebsiella are non-motile while the other Enterobacteriaceae have flagella and, in general, are motile;

b) their capsule: K. pneumoniae and E. aerogenes produce capsules and give rise to large mucoid colonies:

c) their utilization of specific substrates as the sole carbon source -- e.g., various sugars or citrates; and

d) their production of characteristic end products -- e.g., indole from tryptophane, ammonia from urea, H₂S, etc.

Enteric organisms NOT included in this family are

1) anaerobes -- see exercise on anaerobes.

2) Acinetobacter -- oxidase positive; obligatively aerobic.

a. A. anitratus does not ferment but oxidizes carbohydrates.

b. A. lwoffi neither ferments nor oxidizes carbohydrates.

3) Alcaligenes -- oxidase positive; facultatively anaerobic; neither ferments nor oxidizes carbohydrates.

4) Pseudomonas -- oxidase positive, obligatively aerobic; does not ferment but oxidizes carbohydrates.

5) Vibrio -- oxidase positive; strongly aerobic; ferments carbohydrates.

6) Aeromonas and Plesiomonas

Escherichia: This Enterobacteriaceae genus is widely distributed in nature. E. coli is sometimes referred to as the colon bacillus, because it is the predominant facultative species in the large intestine of man. Its presence in a water supply, in milk, or at a beach indicates fecal contamination and raises the specter of a typhoid fever epidemic or an outbreak of gastroenteritis. E. coli is the most common cause of urinary tract infections and may also produce puerperal sepsis, appendicitis, peritonitis, endotoxic shock, wound infections, traveler's diarrhea and infantile diarrhea.

Haemophilus. Haemophilus species are part of the normal gram-negative flora of the upper respiratory tract. The major pathogenic species of Haemophilus is H. influenzae. H. parainfluenzae, H. hemolyticus and H. parahemolyticus are not frequently encountered. H. hemlyticus is a diagnostic nuisance, since it rarely produces disease but may look like Group A Streptococcus on rabbit BAP. H. ducreyi causes chancroid -- a venereal disease with lesions resembling the chancre of syphilis. Chancroid is rare in Kentucky.

H. influenzae produces serious infections: in children it causes epiglottitis (croup), laryngitis, sinusitis, bronchiolitis, pneumonia, meningitis. This pathogen is an important cause of bacterial meningitis in children aged 2 mo. to 5 yrs. In adults H. influenzae causes chronic pulmonary disease and can cause pneumonia. H. influenzae commonly causes infection following a prior infection that alters lung defenses (e.g., viral influenza) -- i.e., it is a common secondary invader. An important virulence property of H. influenzae is the carbohydrate capsule. H. influenzae isolates are divided into 6 antigenic serotypes based on the capsule. "Nontypable" H. influenzae (= nonencapsulated) are common members of normal nasopharyngeal flora in everyone except neonates. These also can cause local infections such as otitis media, bronchitis, and pneumonia.

All strains appear as minute Gram negative rods, which are often thread-like or pleomorphic (i.e., assume many different shapes). Some strains may form small coccobacilli with blue-purple bipolar staining and may be mistaken for encapsulated pneumococci . Haemophilus sometimes grow as chains, again resembling pneumococci or streptococci. In old cultures autolysis yields irregular staining and much amorphous debris; hence the importance of Gram staining young cultures -- i.e., 18-24 hours old.

The genus name Haemophilus derives from the fact that these species need various growth factors contained in blood; hence these organisms are termed "hemophilic", meaning "to love blood". Different species of Haemophilus require one or both of two factors found in blood. These factors are designated X (hematin = hydroxyhemin or hemin = chlorohemin) and V (NAD/NADP). H. influenzae and H. hemolyticus require both hemin (X) and NAD (V), whereas H. parainfluenzae and H. parahemolyticus require only NAD (V).

Helicobacter: H. pylori , a gram-negative rod, causes duodenal ulcers in humans and has been implicated as a predisposing factor in stomach cancer. It is closely related to Campylobacter.

Klebsiella: This Enterobacteriaceae genus is part of the normal GI flora but in other areas of the body may be a serious pathogen.

K. pneumoniae and K. oxytoca are the most important human pathogens of this genus. K. pneumoniae is found in the respiratory tract and feces of 5-10% of healthy persons. But in patients with chronic pulmonary disease it may cause pneumonia and produce chronic lung abscesses requiring surgical resection. The organism also causes urinary tract infections. Its invasiveness, like that of pneumococcus, depends on the antiphagocytic property of its capsule. The presence of a capsule is reflected in the colony being large & mucoid.

K. oxytoca is a species recently isolated at UKMC in clinical situations like those of K. pneumoniae.

Morganella -- see Proteus.

Mycobacterium. Three mycobacterial species are highly virulent for humans: M. tuberculosis, M. bovis, and M. leprae. M. tuberculosis and M. bovis cause tuberculosis (TB) or TB-like illness. BCG is an avirulent M. bovis that is used as a live vaccine in some countries. M. leprae causes leprosy. Other mycobacterial species live in soil, birds, or as members of our normal flora. Some of these cause skin or respiratory infections in people and are particularly dangerous opportunists in AIDS patients. As a group, they are termed MOTT (mycobacteria other than M. tuberculosis [really is other than M. tuberculosis, M. bovis, and M. leprae]). The species causing most human disease are: M. kansasii, M. avium, and M. intracellulare. They are opportunists that cause tuberculosis-like disease in immunocompromised patients (e.g., AIDS patients). It is hard to distinguish M. avium from M. intracellulare, and they are grouped together as the M. avium-intracellulare complex (MAC). Others less frequently encountered are M. scrofulaceum (lymphadenitis) and M. fortuitum, M. marinum, M. ulcerans, and M. chelonei (all three of which cause skin lesions). M. smegmatis is found on surface of human genitals as a member of our normal flora.

Mycobacteria are Acid fast (AF): they stain poorly by the Gram procedure, because the complex waxy wall containing mycolic acid limits accessibility of Gram stains. However, carbol fuchsin stain (pink-red), once infused into the wall, resists destaining with ethyl alcohol containing 3% HCl, causing the bacteria to be "acid-fast" in their staining. The wall confers hardiness to the mycobacteria: they are very resistant to desiccation. This is important for transmission of mycobacteria in droplet nuclei and in dust.

Mycobacteria grow very slowly: some (e.g., M. tuberculosis) take as long as 24 h to double in the laboratory; others are faster, but still very slow compared to other bacteria. Clinical samples are incubated up to 8 weeks; sputum that has too few bacteria to be seen in typical smears can give rise to positive cultures.

M. leprae is an obligate intracellular parasite: it cannot be grown outside of mammalian cells. This organism is very slowly growing, with a generation time of 12-13 days. It grows best at temperatures below 37^oC; hence, it sets up infections in superficial tissues.

M. tuberculosis, M. bovis and MOTT are facultative intracellular pathogens. Their aerobic nature favors well-aerated tissues (lung apices), but they can and do set up infection in many organs in disseminated miliary tuberculosis.

In the laboratory, the MOTT are divided into 4 Runyon groups based on their growth rate and pigmentation when incubated in the dark or light. The optimal growth temperature also is diagnostically helpful (M. marinum and M. ulcerans don't grow well at 37^oC; they need a temperature around 30^oC).

Runyon Group I are photochromogens: yellow to orange when grown in light but nonpigmented when grown in dark. Examples: M. kansasii and M. marinum.

 

Runyon Group II are scotochromogens: yellow to orange when grown in dark or light (get darker in light). Example: M. scrofulaceum.

 

Runyon Group III are nonchromogens: nonpigmented whether grown in light or dark. Example: M. avium-intracellulare.

Groups I through III are all slow-growing.

 

Runyon Group IV are rapid growers (growth seen within 1 week): M. fortuitum, M. chelonei, M. smegmatis.

Neisseria. Two serious human pathogens belong to the gram-negative Neisseria genus: N. gonorrhoeae, the gonococcus, causes gonorrhea and pelvic inflammatory disease; N. meningitidis, the meningococcus, causes meningitis and meningococcemia. Neisseria are parasites of human mucous membranes. The gonococcus is a pathogen only of humans and is carried in a human reservoir, mainly asymptomatic or ignored symptomatic people of ages 18-30. The sole reservoir for meningococci is the oropharynx of humans. 5-10% of adults are nasopharyngeal carriers. For N. meningitidis, the capsule is an important virulence property. There are 8 antigenic groups of N. meningitidis identified on the basis of capsular antigens: A, B, C, D, X, Y, Z, and Z'. Those in groups A, B, and C cause most of the human infections. (In contrast, N. gonorrhoeae express little or no capsule.)

Neisseria are Gram-negative, nonmotile cocci. They usually appear as pairs (Gram negative diplococci; GNDC) with apposing surfaces flattened or indented, giving a "kidney-bean" shape ( ). All Neisseria are oxidase positive and catalase positive. Neisseria can carry out both aerobic and anaerobic respiration. They require 37^oC and elevated CO₂, and lose viability rapidly when conditions depart from these; hence, specimens are rushed to the lab (swabs are inoculated into transport medium or onto a plate at bedside) and are NEVER refrigerated. Both gonococci and meningococci are fastidious and require enriched, complex media prepared so as to reduce or eliminate inhibitory factors such as heavy metals and fatty acids. Typical clinical specimens from mucosal surfaces will contain normal flora, including nonpathogenic species of Neisseria, and primary isolation media often include antibiotics to inhibit or limit the growth of these (e.g., in Thayer-Martin medium, useful for oropharyngeal specimens, vancomycin is present to inhibit Gram-positives, colistin [polymyxin E] to inhibit Gram-negative enterics, and nystatin to inhibit yeasts).

 

Gonococci and meningococci must be distinguished from members of the mucosal flora with little pathogenic potential, that grow on primary isolation media: e.g., other Neisseria such as N. sicca and N. lactamica and related genera such as Moraxella catarrhalis. These all are oxidase+ catalase+ Gram- kidney-bean-shaped diplococci. To differentiate these species, the diagnostic lab determines the spectrum of sugars that are oxidized.

 

 

Neisseria sugars: In the following table note the pattern of acid production (+) when the first 3 sugars and the first 5 species are arranged in alphabetical order. NOTE that gonococci metabolize only glucose while meningococci ferment also maltose. N. lactamica can be differentiated from N. meningitidis through the former's oxidation of lactose. (Associate lactamica with lactose.) B. catarrhalis is included in the table because it is a GNDC that resembles Neisseria. It can cause otitis media in children and chronic bronchitis in adults (this latter is being increasingly seen in immunocompromised patients).

 

Neisseria Sugar Reactions
	Glucose	Maltose	Sucrose	Lactose
N. gonorrhoeae	+	0	0	0
N. lactamica	+	+	0	+
N. meningitidis	+	+	0	0
N. sicca	+	+	+	0
N. flavescens	0	0	0	0
Moraxella catarrhalis	0	0	0	0

Nocardia. Nocardia are soil inhabitants that are inhaled or inoculated into skin. They are opportunists and often cause disease in immunocompromized people. There are two clinically important species: N. asteroides (the chief cause of human disease) and N. brasiliensis. N. asteroides causes pulmonary infections called nocardiosis: a diffuse infection with no distinctive symptoms. The organisms can spread via blood to cause abscesses in brain or spinal cord or in subcutaneous tissues. N. asteroides also can cause chronic subcutaneous abscesses (mycetomas) with draining sinuses which discharge "sulfur" granules (colonies embedded in a matrix of calcium phosphate and named for their yellow color). N. brasiliensis causes mycetomas due to inoculation of a breach in skin, typically from soil.

Nocardia are Gram positive; Some are poorly acid-fast ("half acid-fast"). To varying degrees Nocardia have branched filaments, causing them to resemble fungi. They grow more rapidly than M. tuberculosis on media used to culture mycobacteria, and they grow on Sabouraud's medium, an acidic medium useful for culturing fungi. They are aerobic and have a predilection for lung (as with M. tuberculosis); but (also as with M. tuberculosis), they can set up infection in other parts of the body.

Nosocomial infection: any hospital-acquired infection

Opportunist: This is any member of the normal body flora which, while harmless in its normal location may produce disease (1) if it is introduced to another part of the body (e.g., into the urinary tract) or (2) if there are predisposing factors, such as debilitation, neoplasm, or trauma.

Peptococcus - - see Anaerobic cocci

Peptostreptococcus - - see Anaerobic cocci

Porphyromonas - - see Bacteroides

Prevotella - - see Bacteroides

Proteus: These Enterobacteriaceae species are normally found in stool, soil, and sewage. They produce infection when they move out of the GI tract and invade other areas of the body -- e.g., urinary tract, burns, wounds, middle ear, etc. In the diagnostic bacteriology lab they are a nuisance when found in any stool, because they do not ferment lactose. However, they can be differentiated from lac-negative enteric pathogens by their ability to split urea rapidly and to deaminate lysine and phenylalanine. Most strains swarm across the surface of conventional plates, making the identification of Proteus easy but covering up other organisms present. Special techniques are necessary to prevent this swarming in order to isolate these accompanying bacteria. These techniques include

P. mirabilis and P. vulgaris are commonly encountered in the UKMC Bacteriology Laboratory. Members of the Providencia and Morganella genera are commonly present in normal stools and are primarily associated with urinary tract infections. These organisms closely resemble Proteus biochemically and previously were designated as Proteus species. They can be differentiated from Proteus species by their lack of urease.

Providencia rettgeri, Providencia stuartii, and Morganella morganii resemble Proteus species in their disease spectrum but are less common clinical isolates.

P. rettgeri, Morganella and Providencia species are more resistant to most antibiotics than Proteus species.

Providencia -- see Proteus

Pseudomonas: This genus of the gram-negative family Pseudomonadaceae is widely distributed in nature and is present in the GI tract of 10-20% of adults. These organisms are the major agents for the oxidation of organic matter in the soil. They produce some plant diseases. Being psychrophilic, several species cause spoilage of fish and other food under refrigeration. Human infections occur in debilitated patients (particularly cystic fibrosis) and in those having burns or receiving prolonged antibiotic therapy. They frequently become resistant to antibiotics.

Most species are obligate aerobes and do not ferment carbohydrates. Thus on selective plating media they simulate lac-negative pathogens but may be differentiated from them by

Inoculated Kligler's and TSI slants show a K/K reaction with H₂S production. In glucose broth Pseudomonas produces acid oxidatively with gas formation.

On artificial medium, P. aeruginosa often forms a green or blue iridescent pigment (pyocyanin). In some burn infections it produces fluorescein and may be diagnosed directly by the fluorescence of this compound when exposed to U.V. light. P. aeruginosa comprises 99% of Pseudomonas isolates.

A recent problem at UKMC has been the appearance of P. maltophilia, which is resistant to most antibiotics.

Salmonella: Members of this Enterobacteriaceae genus produce a broad spectrum of human diseases ranging from mild cases of food poisoning to fatal septicemias. The typhoid bacillus (S. typhi) has been found so far only in man and duck eggs, while all other species inhabit the intestinal tract of various animals and insects. Cholecystectomy may be necessary to eliminate the chronic carrier state, such as gave lasting fame to Mary Mallon, better known as Typhoid Mary.

A culture which produces a K/A reaction without gas but with H₂S production in a TSI or Kligler's slant and which fails to hydrolyze urea rapidly should be presumptively diagnosed as a strain of S. typhi. Most other Salmonellae produce gas from glucose fermentation but none gives a positive indole test.

Salmonella possess a variety of antigens which form the basis for their serological classification. There are two types of antigens:

1) Somatic or "O" antigens are lipoprotein-polysaccharides associated with the bacterial cell wall. The O antigens, designated by Arabic numerals, provide group identification.

2) Flagellar or "H" antigens are proteins associated with the flagella. These antigens, designated by small letters and Arabic numerals, provide specific type identification.

Serratia: Cultures of red pigmented strains of Serratia marcescens have been used in aerosols to study settling and drifting of bacteria in air currents. The stigmata reported on religious statues and paintings were possibly due to this red pigment-producing strain. While initially regarded as a harmless saprophyte, in the last decade Serratia have been isolated increasingly from human infections -- e.g., empyema, septicemia, urinary tract infections, post operative wound infections, etc. Infections of arterial grafts by Serratia have been reported.

Most of these gram-negative strains are lac-negative or ferment lactose slowly; at least 3/4 of all strains so far isolated are non-pigmented. Most pigment-producing strains are temperature sensitive, doing so only at room temperature.

Shigella: This Enterobacteriaceae organism produces so-called "bacillary dysentery" in man. Unlike Salmonella infections, which often spread via the bloodstream, Shigella species rarely produce a bacteremia. Biochemically, Shigella differs from Salmonella in being non-motile, in fermenting glucose without gas, and in not forming H₂S.

S. flexneri and S. sonnei are the most commonly isolated species in the U.S.

Staphylococcus. Members of this genus are found normally on the skin, nasal mucosa, and other mucous membranes. The most important species are S. aureus, which is frequently associated with purulent infections, (e.g., pimples, furuncles, abscesses, empyema, osteomyelitis, etc.), and S. epidermidis, which is usually present as an innocuous saprophyte.

Staphylococcus is a very, very common cause of infectious disease: more than 80% of suppurative disease seen by physicians is due to staphylococci. Staphylococcus is an important cause of nosocomial (= hospital-acquired) infection (second to Escherichia coli). The organisms are spread by hospital personnel to patients, who present an especially susceptible (compromised) population.

Staphylococcal disease can be divided into 2 main types.

 

1. Suppurative = accumulation of pus. This class accounts for the majority of "staph" infections. Abscesses are formed in wide variety of tissues. These can be surface-located: e.g.: furuncle [= boil]; carbuncle [multiple, interconnected boils]; pimple; stye [infected sebaceous gland on eyelid]; cellulitis [diffuse inflammation] such as impetigo; mastitis [breast infection]. Staph also can spread (metastasize) from a surface location to deep tissues and cause abscesses, e.g.: osteomyelitis [abscess in bone], peritonitis, meningitis, pneumonia.

 

2. Toxic (toxemia) = disease due to toxins released by the bacteria. This can affect one organ system, e.g.: gastroenteritis [food poisoning] (GI system); scalded skin syndrome and Ritter's disease (skin) or have a multisystemic effect, as in toxic shock syndrome.

Staphylococci are Gram positive and characteristically appear as clusters of cocci in stained smears taken from solid media and from suppurative infections. However, in liquid media and in clinical specimens harboring a normal light staphylococcal flora the smear may reveal short chains containing 4 cocci or less. All strains are facultative anaerobes. All staphylococci possess the enzyme catalase. They survive in clinical specimens for long periods or under conditions in which other organisms are lost.

The following tabulation summarizes the main features distinguishing S. aureus from S. epidermidis:

S. aureus	S. epidermidis
Pigment production: On BAP most colonies are golden-yellow.	On BAP most colonies are porcelain-white or colorless.
Hemolysis: Many strains are beta-hemolytic.	Rarely hemolytic.
Mannitol fermentation: Ferment mannitol with acid production.	Occasionally mannitol-positive.
Coagulase test: All strains of S. aureus are by definition coagulase positive.	Always negative.

Streptococcus Members of the genus Streptococcus make up a large fraction of the bacterial flora of the mouth and pharynx and also constitute part of the normal stool, including both its aerobic and anaerobic flora. Some strains produce serious human infections ranging from tonsillitis to puerperal sepsis. Still other strains have great economic importance, for their fermentation of sugars to lactic acid is responsible for cheese, sauerkraut, yogurt, and silage.

Medically important species are listed below with the clinically most commonly used designation in capital letters. The group designations are based on the presence of group-specific cell wall carbohydrate antigens. Groups A and D are most frequently associated with human disease; Group B is an important cause of neonatal sepsis and meningitis. Others are occasional causes of disease in humans. Streptococci other than group A are opportunists: they become invasive when displaced from their normal site (e.g., by wound or instrumental procedure) or when the body's defenses are compromised by underlying disease or by insufficient development (neonate).

1. GROUP A STREPTOCOCCUS = Streptococcus pyogenes: major pathogen of humans e.g., "strep throat" (pharyngitis), acute rheumatic fever, glomerulonephritis, skin diseases (erysipelas, impetigo [= pyoderma]).

2. GROUP B STREPTOCOCCUS = S. agalactiae: often in normal flora of vagina, GI tract, pharynx; causes UTI in women, neonatal meningitis and sepsis, puerperal sepsis, skin infection, endocarditis.

3. GROUP C STREPTOCOCCUS = S. equisimilis: can be present in upper respiratory tract of farm animals and humans; may cause human infections (e.g., UTI, wound infections, endocarditis, pharyngitis, brain abscess, puerperal sepsis). The Group C Streptococcus is source of streptokinase used to dissolve clots in heart attack patients.

4. GROUP D STREPTOCOCCUS = ENTEROCOCCI and NON-ENTEROCOCCI: normal flora of skin, upper respiratory tract, GI tract, GU tract; UTI, wound infections, infectious endocarditis.

5. GROUPs E-V STREPTOCOCCUS: no Linnaean designations yet; Group G occasionally cause cellulitis, infections of bone or joint.

6. VIRIDANS STREPTOCOCCI = e.g., S. mutans, S. salivarius, S. sanguis; are nongroupable (i.e., they belong to a heterogeneous collection of species that do not contain group-specific antigen); normal flora of nasopharynx, mouth; also found in GI tract, female genital tract; cause dental caries (S. mutans) and gum disease. S. sanguis is the most frequent bacterial cause of infectious endocarditis; other viridans streptococci cause endocarditis, wound and other infections.

7. ANAEROBIC (or microaerophilic) STREPTOCOCCI = e.g., peptostreptococci; abscesses in lung and brain, gingivitis, mixed infections -- i.e., where O₂ is low.

8. S. PNEUMONIAE: pneumonia. This organism is an innocuous inhabitant of the pharynx in 40-70% of normal people at different times. It may produce pyogenic infections upon invading the sinuses or middle ear or upon passing the epiglottal barrier -- as during a viral upper respiratory infection. Any of these infections may progress to bacteremia or meningitis. Virulence of the pneumococcus is determined in part by its capsular polysaccharide.

All species of Streptococcus are Gram-positive cocci in short or long chains. Diplococcal clusters may be seen in smears taken from solid media. Streptococci lack catalase. Most strains are facultative anaerobes; some are obligate anaerobes (i.e., anaerobic streptococci).

The following properties distinguish among species of Streptococcus:

The pneumococcus (S. pneumoniae) is distinguished from other streptocci by the following characteristics:

Vibrio: These are short, slightly curved (comma shaped), motile gram-negative rods of the family Spirellaceae. In contrast to the Enterobacteriaceae, Vibrio organisms are oxidase-positive and grow well in media which are too alkaline (pH 9) for most other bacteria. Vibrio are sensitive to acid and are killed in mixed cultures growing on fermentable sugars.

V. cholerae is the cause of cholera, endemic in India for centuries. The most serious complication in cholera is the extreme dehydration with circulatory collapse resulting from the voluminous "rice-water" stools. This massive intestinal fluid and electrolyte loss is due to the action of a potent enterotoxin upon the epithelial cells of the small bowel.

V. vulnificus produces serious cellulitis following trauma and exposure to sea water in parts of the world -- e.g. the Gulf Coast of the USA. Patients with severe liver disease may suffer a fatal bacteremia following ingestion of undercooked shellfish (especially oysters) contaminated with V. vulnificus.

V. parahemolyticus causes gastroenteritis.

Veillonella - - see Anaerobic Cocci

Yersinia: Yersinia organisms are oxidase-negative Enterobacteriaceae, which distinguishes them from species currently classified as Pasteurella. Y. enterocolitica infections are primarily from food poisoning and may produce symptoms which mimic acute appendicitis, gastroenteritis, mesenteric adenitis, terminal ileitis, and rarely septicemia. The septicemic form resembles typhoid fever.

Y. pseudotuberculosis is less commonly isolated but causes disease spectrums similar to Y. enterocolitica. Yersinia pestis is the cause of plague, or the "black death"

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