Lecture #63: Microbiology: Bacterial G.I. Infections - NYIT - PDF

Lecture #63: Microbiology: Bacterial G.I. Infections – Normal Flora and Pathogenic E. coli Charles Pavia, Ph.D. Professor; Dept. of Biomedical Sciences [email protected] Office of Academic Affairs  Session Objectives Describe the key roles that epidemiology, microbial virulence...

Lecture #63: Microbiology: Bacterial G.I. Infections – Normal Flora and Pathogenic E. coli Charles Pavia, Ph.D. Professor; Dept. of Biomedical Sciences [email protected] Office of Academic Affairs  Session Objectives Describe the key roles that epidemiology, microbial virulence factors, the normal microbial flora (the so-called “microbiome”) & host-defense mechanisms play in the prevention, or as contributing factors in the development, of gastrointestinal tract infections that are caused by pathogenic bacteria. Describe, based on unique microbiologic features, the normal G.I. bacteria and the pathogenic E. coli that are associated with causing gastrointestinal infections. Describe the various test procedures that are often used in a clinical microbiology lab for the purpose of identifying the pathogenic E. coli bacteria associated with causing gastrointestinal tract infections, and for aiding the clinician in making the correct diagnosis. Source: Course Syllabus  Outline of the lecture Introduction to the bacteria (normal flora?) residing in the G.I. tract (so-called “Microbiome”); Key microbiologically-related properties of the enteric bacteria; Review of the epidemiology of the G. I tract pathogens; Microbiologic and clinical aspects of the pathogenic E. coli A Microbiologic Overview of the G.I. tract The GI Tract & Typical Numbers of Bacteria at the Main Sites ▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄ Composition of the Intestinal Flora of Adult Humans ▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄ ▄ Normal flora of the upper G.I. tract Normal flora of the lower G.I. tract  Key indigenous anaerobic flora Bacteroides fragilis: An anaerobe, but not a strict one (aerotolerant) located in the colon Gram negative rod (or bacillus) having a capsule A common cause of intra-abdominal infections, occurring after internal surgery or a traumatic accident It has LPS, but it lacks endotoxin-like activity, due to the lack of a key phosphate group 10 Bacteroides fragilis: gram stain  Key indigenous anaerobic flora cont’d: Clostridium difficile: An anaerobic, spore-forming, toxigenic gram-positive bacillus It causes antibiotic-associated diarrhea and colitis Pts develop pseudomembranous colitis following the use of various antibiotics, such as the cephalosporins and fluoroquinolones (often given to pts during & after surgery) C. diff. produces two toxins – A & B (both are potent cytotoxins) Recently implicated as a serious nosocomial pathogen: release of spores by patients colonized with C. diff. may colonize/infect other patients C. diff.-associated pseudomembranous colitis Endoscopic appearance of the sigmoid colon showing diffuse pseudomembranes with some hemorrhaging. 13 Gram stain of C. difficille showing the pale- staining endospores  Key Properties of Enteric Bacteria 15  Key Properties of Enteric Bacteria cont’d – Lactose fermentation: E.coli – most strains ferment this sugar which often helps in their identification from a patient sample. MacConkey agar – most frequently used type of agar for measuring sugar (lactose) fermentation patterns of E. coli and other gram-negative enteric bacteria. Special note: not all gram-negatives will grow on MacConkey agar: e.g., Haemophilus and Neisseria species; also, all gram-positive bacteria will NOT grow on MacConkey agar due to special inhibitors that have been incorporated into the agar. 17 Growth on MacConkey agar: red or purple-colored colonies indicate lactose-fermentation; amber or colorless colonies indicate non-lactose fermenters Non-lactose Lactose- fermenter fermenter 18 MacConkey agar: differential appearance of lactose fermenters (black arrow) and those that do not ferment lactose (blue arrow) ← →  Key Properties cont’d Genetic properties: Chromosomal vs. plasmid DNA: no nucleus Bacteria can exchange or transfer DNA via: conjugation – mating via pili transduction – phage (virus)-mediated transformation – “free” DNA [mostly done experimentally in a lab, but probably occurs elsewhere] Phage-typing – unique for a particular species or type of bacterium: a specific virus recognizes & infects only one type of bacterium An Image of a Bacteriophage  Now for a Refresher on some key epidemiologic terms and concepts that have much relevance when it comes to G.I. tract infections.  What is a localized epidemic? This occurs when a disease outbreak affects a small group of people (usually