Pathogenic Bacteria: Staphylococci and Streptococci

Staphylococci are gram-positive cocci found widely in nature
Three medically important species are Staph. aureus, Staph. epidermidis, and Staph. saprophyticus
- Staph. aureus is the most important, differentiated by being coagulase-positive and mannitol-positive, and causing blood agar hemolysis
They are considered potentially pathogenic
All Staphylococci are catalase-positive, unlike Streptococci which are catalase-negative
Staphylococci produce intracellular (endo) pigments which contribute to coloration
Staphylococci cause acute toxic food poisoning and toxic shock syndrome

Staph. epidermidis: non-pigmented, non-hemolytic colonies, commonly hospital-acquired
Staph. saprophyticus: colonies are white or yellow, non-hemolytic, urinary tract infections.

Gram-positive cocci arranged in chains, of variable length; widely distributed and non-spore-forming
An important characteristic for identifying streptococci is the type of hemolysis they produce on blood agar
Beta-hemolytic streptococci produce complete haemolysis with a clear zone. An example is Strepto. pyogenes.
Alpha-hemolytic streptococci produce greenish discoloration due to the production of H2O2 from the breakdown of haemoglobin. Examples include Streptococci viridans and Strepto. pneumoniae.

Strept. pyogenes: sore throat, scarlet fever, toxic shock syndrome
Strept. viridans: part of the normal mouth and throat flora; few species are pathogenic and can be implicated in dental caries, bacteremia, and subacute bacterial endocarditis

Found in the mouth, throat, pharynx, and vagina
Two important pathogens are N. meningitidis and N. gonorrhoeae
Gram-negative cocci arranged in pairs, oxidase-positive
N. gonorrhoeae and N. meningitidis require enriched media for growth
N. meningitidis: causes acute epidemic cerebrospinal meningitis
N. gonorrhoeae: causes gonorrhoea in adult males and vulvo-vaginitis in young girls; a venereal disease

Genus Mycobacterium comprises "acid-fast bacilli" difficult to stain
Cannot be stained by simple stains due to high lipid content
Stained by Ziehl-Neelsen stain, they appear as thin pink rods arranged singly or in small groups
Grow on egg-enriched media such as Lowenstein-Jensen (L-J) medium
M. tuberculosis and M. bovis: cause human tuberculosis
- M. tuberculosis: infects via respiratory route
- M. bovis: infects via oral route (ingestion of infected milk)
Grow very slowly; no growth appears for 2-4 weeks at 37°C
Tuberculin or PPD skin test (Mantoux test): delayed hypersensitivity reaction used for tuberculosis diagnosis
M. Leprae: causes leprosy, affecting mainly the mucous membranes of the nose, skin, and nerve fibers
Leprosy bacillus hasn't been successfully cultured on any medium
Lepromin test

Gram-negative bacilli found in the colon of humans and other animals
Family Enterobacteriaceae: includes many genera
Pathogenic genera include Shigella, Salmonella, Yersinia, and some Escherichia strains.
Opportunistic pathogens include Klebsiella, Enterobacter, Serratia, Citrobacter, Proteus, and Escherichia
Categorized according to lactose fermentation
- Lactose fermenters: Escherichia, Klebsiella, Citrobacter, Enterobacter
- Lactose non-fermenters: Salmonella, Shigella, Yersinia, Proteus

Normal inhabitant of the intestines of humans and animals
Some E. coli strains cause urinary tract infections; produce hemolysis on blood agar
Gram-negative bacilli, grow on simple media
Produce rose-pink colonies on MacConkey's medium due to lactose fermentation

Commonest cause of urinary tract infections
Cause wound infections, appendicitis, bacteremia, and endotoxic shock
Enteropathogenic E. coli (EPEC) cause gastroenteritis in children
Enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and enterohemorrhagic E. coli (EHEC) O157:H7 cause diarrhea in all ages

K. pneumoniae: causes lobar pneumonia in humans
Causes urinary tract infections
K. rhinoscleromatis: causes rhinoscleroma (a granulomatous lesion in the nose and throat)
K. oxytoca: causes hospital-acquired infections

Gram-negative bacilli, non-capsulated
S. Typhi, S. Paratyphi A, B, and C: cause enteric fever
S. Typhimurium, S. Enteritidis, and others cause Salmonella food poisoning or enterocolitis
S. Choleraesuis: causes septicemia with metastatic abscesses
Widal test

Gram-negative bacilli, non-capsulated
Members of the genus Shigella cause bacillary dysentery
Manifestations include abdominal pain, fever, diarrhea with pus, blood, and mucous in stools
Infection leads to local ulcers in the intestine

Gram-negative bacilli, non-capsulated
Proteus organisms are found in the stools of humans and animals
Cause wound infections, urinary tract infections, and septicemia (potentially life-threatening infection with large amounts of bacteria in the blood)
Two important species: Pr. mirabilis and Pr. vulgaris

Gram-negative bacilli, non-spore-forming
P. aeruginosa: the commonest human pathogen in the Pseudomonas group
Causes urinary tract infections, wound infections, corneal ulcers (in contact lens users), pneumonia, sepsis with ecthyma gangrenosum in the skin, osteomyelitis, and endocarditis (in intravenous drug users)

Vibrio cholerae: gram-negative bacilli causing cholera-like disease
Campylobacter: small gram-negative rods; C. jejuni causes 95% of campylobacter enterocolitis, especially in children
Helicobacter pylori: causes gastritis and peptic ulcers; infection is a risk factor for gastric cancer

Gram-negative small coccobacilli
H. influenzae: found in normal throats
Capsulated strains cause meningitis and epiglottitis in children
Also causes complications in respiratory diseases (e.g. bronchitis, pneumonia, sinusitis) in patients with viral influenza