Module 12 Book Notes

  • NOTE: not complete read through chapter/pages assigned!

  • Enterobacteria general characteristics

    • gramn-egative bacilli and coccobacilli.

    • they do not produce cytochrome oxidase except for Plesiomonas

    • they all ferment glucose

    • they reduce nitrate to nitrite except for Photorhabdus and Xenorhabdus

    • they are motile at body temperatures except for Klebsiella, Shigella, and Yersinia

    • except for Klebsiella, Proteus, and some Enterobacter isolates, none has remarkable colony morphology on laboratory media

      • they appear large, moist, and gray on sheep blood agar (SBA), chocolate (CHOC) agar, and most nonselective media

    • microscopic and colony morphology

      • gram-negative, non-spore forming, facultative anaerobes, bacilli

      • may appear coccobacilli or as straight rods

      • no value in using SBA or chocolate agar for ID (except Klebsiella)

        • many E. coli isolates are B-hemolytic

      • best to use methylene blue, MAC agar, and highly selective media for culturing

        • Hektoen enteric (HE)

        • xylose-lysine-desoxycholate (XLD)

          • positive result indicated by color change in agar due to fermentation of present carbohydrate

          • can also distinguish species that produce hydrogen sulfide (H2S)

      • classification

      • virulence and antigenic factors

        • O antigen (somatic antigen) — heat-stable antigen located on the cell wall

        • H antigen (flagellar antigen) — heat-labile antigen on the surface of flagella

        • K antigen (capsular antigen) — heat-labile polysaccharide found in some encapsulated species

      • clinical significance

        • ubiquitous in nature

        • reservoir: GI tract; some live in water, soil or sewage; plant pathogens

        • opportunistic pathogens

          • part of usual intestinal flora in humans and animals

          • harmful when moved to another body site

        • primary pathogen

          • bacteria introduced from environment to host by contaminated soil or water; also devastating in the immunocompromised

      • antimicrobial resistance

        • three categories - urgent, serious, concerning

          • carbapenemresistant Enterobacteriaceae (CRE) prevalent in clinical settings

            • resistance to imipenem, meropenem, doripenem, or ertapenem, or documentation that the isolate possesses a carbapenemase

          • plasmid-transmittable colistin marker mcr-1

            • allows pathogen to exhibit more resistance to a variety of antibiotics

  • Escherichia coli as an opportunistic pathogen

    • can cause UTI’s diarrheal diseases, central nervous system infections, bacteriuria, septicemia, neonatal sepsis, and meningitis

    • most strains are motile and have adhesive fimbriae and sex pili

      • they may also have O, H, and K antigens

    • colony morphology on selective and differential media, like MAC or EMB agar

      • lactose-positive (pink) with surrounding precipitated bile salts on MAC agar

      • green metallic sheen on EMB agar

    • general associations of E. coli

      • Fermentation of glucose, lactose, trehalose, and xylose

      • Production of indole from tryptophan

      • Glucose fermentation by the mixed acid pathway: methyl red positive and Voges-Proskauer negative

      • Does not produce H2S, deoxyribonuclease (DNase), urease, or phenylalanine deaminase

      • Cannot use citrate as a sole carbon source

    • uropathogenic E. coli

      • cause of UTI’s from indigenous biota in the large intestine

        • virulence factors present that allow them to attach to urinary epithelial mucosa by the use of pili

          • cytolysins & aerobactins also contribute

            • cytolysins kill immune effector cells and inhibit phagocytosis & chemotaxis of certain WBC

            • aerobactin allows the bacterial cell to chelate iron; free iron is generally unavailable within the host for use by bacteria

      • can also cause acute pyelonephritis

    • gastrointestinal pathogens

      • five major categories of diarrheagenic E. coli

        • enterotoxigenic Escherichia coli (ETEC)

        • enteroinvasive Escherichia coli (EIEC)

        • enteropathogenic Escherichia coli (EPEC)

        • enterohemorrhagic Escherichia coli (EHEC)

        • enteroadherent

          • includes: enteroaggregative Escherichia coli (EAEC) and diffusely adherent Escherichia coli (DAEC)

    • enterotoxigenic Escherichia coli

      • ETEC strains that cause traveler’s diarrhea which is spread by contaminated food or water

        • self-limiting; symptoms experienced are watery diarrhea, abdominal cramps, and sometimes nausea, usually with no vomiting or fever

      • transmission of disease by poor hygiene, reduced availability of potable water sources, and inadequate sanitation

        • stomach acidity as a protective mechanism to inhibit colonization and initiation of disease

      • high infective dose — 106-1010 required

      • ETEC strains can produced one or both of two toxins into the small intestine

        • a heat-labile toxin (LT) ~ similar in action and amino acid sequence to cholera toxin

          • AB toxin in which B subunit facilitates entry into cell & A subunit activates cellular adenylate cyclase (increase in cAMP)

          • this results in hypersecretion of electrolytes and fluids into the intestinal lumen

        • a heat-stable toxin (ST) ~ stimulates guanylate cyclase and results in cGMP abundance which causes hypersecretion