Microbiology Tutorial

Bacterial Infections

How do bacteria cause disease? Here are some examples:

• Growth in tissues destroys the tissues from release of proteolytic enzymes

• Streptococcus - erysipelas

• Clostridium perfringens - gas gangrene [Identification]

• Release of exotoxins

• Vibrio cholerae - severe watery diarrhea

• Corynebacterium diptheria - growth promotes elaboration of a myocardial toxin [Identification]

• Staphylococcus aureus - acute self-limited diarrhea [Identification]

• Release of endotoxins

• gram negative bacteria such as E. coli and Klebsiella are most prone to do this, because the lipopolysaccharide (LPS) in the cell wall is a potent endotoxin.

• attract neutrophils, cause fever, can produce shock

• Antigens are similar to host antigens, generating immune reaction: Group A Streptococcus producing post-streptococcal glomerulonephritis and rheumatic heart disease

Appearances

• Microscopic: primary cellular response is neutrophils (chemotactic factors include C5a, leukotrienes, HPETE) which is non-specific and destructive

• Gross: redness, swelling, purulent exudate (pustule, abscess), necrosis

• Clinical: fever (interleukin), pain (bradykinin, prostaglandin), increased WBC count

Algorithms for Laboratory Diagnosis

• Gram positive algorithm

• Gram negative algorithm

Sites of infection and organisms involved

• Skin infection

• Staphylococcus aureus - impetigo [Identification]

• Streptococcus group A - often due to hemolysins, toxins, and streptolysins elaborated by the bacteria. Lancefield typing of these organisms (based on cell wall polysaccharide, or lipoteichoic acid moiety, antigens) can be performed for epidemiologic purposes [Identification]

• impetigo - pustules

• scarlet fever - pharyngitis with rash

• erysipelas - "cellulitis" or spreading infection in dermis

• Vibrio vulnificans - necrotizing skin infection with sepsis

• Respiratory tract

• Lower respiratory tract - characterized by pneumonia

• Streptococcus pneumoniae [Identification]

• Staphylococcus aureus [Identification]

• Klebsiella pneumoniae [Identification]

• Pseudomonas aeruginosa [Identification]

• Hemophilus influenzae [Identification]

• Legionella pneumophila [Identification]

• Upper respiratory tract - characterized by obstruction, cough

• Bordetella pertussis - whooping cough produced by bacterial exotoxin, rare since vaccine use [Identification]

• Corynebacterium diptheriae - diptheria characterized by an obstructive pseudomembrane of ulcerated necrotic epithelium along with production of an exotoxin that damages heart [Identification]

• Gastrointestinal tract

• Salmonella typhi - typhoid fever with longitudinal ulcers of small intestine (inflammation with lymphocytes, plasma cells, and macrophages), neutropenia, and splenomegaly [Identification]

• Vibrio [Identification]

• Vibrio cholerae - florid diarrhea from non-invasive organisms producing enterotoxin that activates adenylate cyclase

• Vibrio parahemolyticus - infection is associated with seafood consumption, particularly in Florida. It occasionally causes septicemia

• Vibrio vulnificus - can cause septicemia and death following ingestion of seafood or from contamination of wounds in a marine environment. V. vulnificus is the leading cause of death in the US related to seafood consumption, particularly raw Gulf Coast oysters

• enteropathogenic E. coli - "traveler's diarrhea" - enterotoxin produces diarrhea similar to, but not as severe as, cholera; can be life-threatening in infants and children

• Shigella - bacillary dysentery from superficially invasive bacteria with ulceration of colon [Identification]

• Helicobacter pylori - gastritis

• Clostridium difficile - pseudomembranous colitis from enterotoxin when normal gut flora diminished by antibiotic therapy [Identification]

• Bacteroides fragilis - intra-abdominal sepsis [Identification]

• Food poisoning in the buffet line

• Staphylococcus aureus - enterotoxin - vomiting and diarrhea hours after ingestion [Food]

• Salmonella - diarrhea and cramps 1 to 2 days after ingestion of bacteria [Identification] [Food]

• Clostridium botulinum - botulism with paralysis from neurotoxin in poorly prepared food

• Bacillus cereus - vomiting within hours of ingesting foods in which the spores germinate and grow readily, such as reheated fried rice [Identification] [Food]

• Urinary tract

• Proteus vulgaris or mirabilis [Identification]

• Providencia stuartii[Identification]

• E. coli [Identification]

• Other gram-negative enteric organisms such as Enterobacter [Identification]

• Enterococcus [Identification]

• Genital/congenital

• Neisseria gonorrheae - gonorrheal infection [Identification]

• Streptococcus - group B strep with stillbirth and neonatal infection [Identification]

• Listeria monocytogenes - chorioamnionitis with abortion, stillbirth, or neonatal sepsis with meningitis [Identification]

• Heart (endocardium): infection arrives from bloodstream and settles on valve, producing a vegetation; aided by presence of an anatomically abnormal valve or a large inoculum (intravenous drug use)

• Staphylococcus aureus or Staphylococcus epidermidis [Identification]

• Streptococcus viridans group [Identification]

• Pseudomonas aeruginosa [Identification]

• Meninges: infection arrives from bloodstream, from sinuses, or via direct trauma

• E. coli - neonatal [Identification]

• Streptococcus group B - neonatal [Identification]

• Hemophilus influenzae - in children [Identification]

• Neisseria meningitidis - starts inocuously in nasopharynx, can lead to disseminated intravascular coagulopathy (DIC) and adrenal hemorrhage and necrosis (Waterhouse-Friedrichson syndrome) [Identification]

• Streptococcus pneumoniae - in older persons [Identification]

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