Microbiology 10: Introduction to Pathogenic Bacteria
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Staphylococci and streptococci both Gram + cocci
Escherichia coli is Gram – enteric bacteria
Tubercle bacillus (Mycobaterium tuberculosis) is acid-fast bacteria
2 large categories
o “typical” bacteria – rods and cocci (Gram + and -)
o Those not in first category
Isolates from different hosts of 1 specis = strains
o E. coli strain in research = K12
“Typical” Bacteria
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Gram stain = presence of outer membrane in Gram - bacteria and murein layer in Gram +
bacteria
o Subdivided into rods or cocci
o Gram + -> appear dark violet; Gram - -> appear red
o Gram + cocci and Gram – rods are most common infections
Gram-Positive Cocci
o Streptococci
 Chains of spherical cells (strings of pearls)
 β-hemolytic streptococci -> lyse RBCs with clear area around
 main pathogens (A through T)
o group A strep = S. pyogenes (cause strep throat, infections with
rheumatic fever or glomerulonephritis)
 α-hemolytic streptococci -> green around colony
 S. pneumoniae (bacterial pneumonia)
 Some strains contribute to production of cheese
 Do not carry out respiration only fermentation
 Most are oxygen-tolerant anaerobes
o Staphylococci
 S. aureus and S. saprophyticus look like bunches of grapes
 Withstand many chemical and physical agents
 Aerobes
 Found esp on skin (pus, osteomyelitis, endocarditis)
 Secrete coagulase
Gram-Negative Cocci
o Neisseria -> genus includes gonococcus (gonorrhea) and meningococcus (meningitis and
severe septicemia)
o Outer membranes contain endotoxin (lipopolysaccharide)
Gram-positive Rods
o Abundant in environment
o Diphtheria -> Corynebacterium diphtheriae (relatives are diphtheroids)
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 Common inhabitant of skin and mucous membranes
o Most common -> spore-forming rods (large)
 Aerobic Bacillus anthracis
 Strict anaerobes (Clostridium)
 C. botulinum (botulism), C. tetani (tetanus), C. perfringens (gas
gangrene)
 Symptoms caused by exotoxins
 C. difficile (pseudomembranous colitis from antibiotic use)
 Listeria monocytogenes (immunocompromised and pregnancy
infection)
Gram-negative Rods
o Enteric bacteria = Echerichia coli, Salmonella (typhoid fever, food poisoning), Shigellea
(bacillary dysentery)
 Do not for spores, many are motile
 Some ferment lactose (E. coli and others) and some don’t (Shigella and
Salmonella)
 Distant relatives = Vibrio, Pseudomonas (found in aqueous environments),
Campylobacter jejuni (infectious diarrhea), Helicobacter pylori (gastritis, gastric
ulcer/cancer)
o Fastidious and Small Gram-negative rods = Haemophilus (pneumonia and meningitis),
Bordetella (whooping cough), Brucella (brucellosis), Francisella (tularemia), Bartonella
(cat scratch fever), Legionella (in soil, water)
o Strictly Anaerobic Gram-negative Rods = Bacteroides (intestinal flora, stimulus for
proper tissue development)
 May be harmful if deposited into deep tissues (peritonitis)
Not So Typical Bacteria
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Acid-Fast Bacteria
o Synonymous with Mycobacterium (tubercle bacillus [M. tuberculosis] and leprosy
bacillus [M. leprae])
o Withstand many chemicals (waxy envelope)
 Penetrated by dyes if bacteria heated or treated with detergent = Ziehl-Neelsen
technique (red dye fuschsin with detergents + 3% HCl + blue dye)
 “Red bugs” visible against blue background
o Environmental species = atypical acid-fast bacilli (Mycobacterium avium-intracellulare)
o Sometimes form branches like fungi (myco)
 Relatives -> Nocardia [aerobic] and Actinomyces [anaerobes] and Streptomyces
(make antibiotics [streptomycin, tetracycline, etc])
Spirochetes
o Helical (spring)
o Treponema pallidum (syphilis) -> do not dye
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Leptospira (icterohemorrhagic fever), Borrelia recurrentis (relapsing fever), Borrelia
burgdorferi (Lyme disease)
Chlamydiae
o Intracellular bacteria (don’t grow on media) -> in phagocytes
o Chlamydia trachomatis (STD), C. pneumonia (pneumonia, atherosclerosis)
Rickettsiae
o Intracellular obligate parasites, rod-shaped
o Cause typhus and Rocky Mountain spotted fever
 Transmitted by bite of arthropod (except Coxiella burnetii [Q fever] inhaled)
o Ehrlichia (infect WBCs)
Mycoplasmas
o Lack rigid cell wall (aka murein)
 Resistant to penicillin
 Resemble regular L bacteria
o Need sterols for nutrition
o Mycoplasma pneumoniae, Ureaplasma ureae
o Grow in hypertonic mediums
Tropheryma whipplei (Whipple disease) -> diarrhea and intestinal bleeding, lymphadenopathy
o IDed by PCR and 16S rRNA analysis