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The Enterobacteriaceae Basic Properties

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1 The Enterobacteriaceae Basic Properties
Dr. John R. Warren Department of Pathology Northwestern University Feinberg School of Medicine June 2007

2 Characteristics of the Enterobacteriaceae
Gram-negative rods Glucose is fermented with strong acid formation and often gas Cytochrome oxidase activity is negative Nitrate is reduced to nitrite

3 Gram’s Stain for Bacterial Morphology
Crystal violet binds to cell wall peptidoglycan with Gram’s iodine as a mordant Safranin or basic fuchsin counterstains bacterial cells decolorized by alcohol-acetone

4 Gram’s Stain for Bacterial Morphology
Thick cell-wall peptidoglycan layer of gram-positive bacteria strongly binds crystal violet and resists decolorization by alcohol-acetone Thin cell-wall peptidoglycan layer of gram-negative bacteria located beneath a thick lipid-rich outer membrane weakly binds crystal violet that is readily removed by alcohol-acetone decolorization

5 Gram’s Stain Procedure
Flood surface of smear with crystal violet solution After 1 min thoroughly rinse with cold tap water Flood smear with Gram’s iodine for 1 min Rinse smear with acetone-alcohol decolorizer until no more crystal violet in rinse effluent Rinse with cold tap water Flood smear with safranin (regular Gram’s stain) or basic fuchsin (enhanced Gram’s stain) Dry smear in slide rack Microscopically examine stained smear using oil-immersion light microscopy

6 Glucose Fermentation Oxidation-reduction of glucose in the absence of molecular oxygen (anaerobic glycolysis) Energy from hydrolysis of chemical bonds in anaerobic glycolysis captured as high energy phosphate bonds of adenosine triphosphate (ATP) NAD is reduced to NADH2 by accepting electrons during glycolytic conversion of glucose to pyruvate NADH2 in turn reduces pyruvate with oxidation of NADH2 to NAD which supports continued anaerobic glycolysis, and generation from pyruvate of alcohols, carboxylic acids, and CO2 gas End products of glucose fermentation: organic acids and CO2 gas Fermentation detected by acidification of glucose-containing broth (color change in broth or agar medium containing pH indicators), and (for aerogenic species) production of gas (fractures in agar, gas bubbles in inverted Durham tube) pH indicators: phenol red (yellow at acid pH), methyl red (red at acid pH), neutral red (red at acid pH), bromcresol purple (yellow at acid pH)

7 Spot Cytochrome Oxidase Test
The spot cytochrome oxidase test is the first test performed with gram-negative bacteria recovered in culture The optimal plate medium for a spot cytochrome oxidase test is a trypticase soy agar (TSA) containing 5% sheep blood Bacterial colonies should be 18 to 24 hr old

8 Spot Cytochrome Oxidase Test
In a positive test, bacterial cytochrome oxidase oxidizes the colorless reduced substrate tetramethyl-p-phenylenediamine dihydrochloride (TPDD) forming a dark purple oxidized indophenol product Streak a small portion of bacterial colony to filter paper soaked with a 1% solution of TPDD If the streak mark turns purple in 10 sec or less, the spot oxidase test is interpreted as positive

9 Nitrate Reduction Enterobacteriaceae extract oxygen from nitrate (NO3) producing nitrite (NO2) NO2 detected by reaction with α-naphthylamine and sulfanilic acid producing a red colored complex Absence of red color indicates either no reduction of NO3 or reduction to products other than NO2 (denitrification) Confirmation of true negative test: addition of zinc ions which reduce NO3 to NO2 producing a red color in the presence of α-naphthylamine and sulfanilic acid

10 Enterobacteriaceae: Genetic Properties
Chromosomal DNA has 39-59% guanine-plus-cytosine (G+C) content Escherichia coli is the type genus and species of the Enterobacteriaceae Species of Enterobacteriaceae more closely related by evolutionary distance to Escherichia coli than to organisms of other families (Pseudomonadaceae, Aeromonadaceae)

11 Enterobacteriaceae: Major Genera
Escherichia Shigella Salmonella Edwardsiella Citrobacter Yersinia Klebsiella Enterobacter Serratia Proteus Morganella Providencia

12 Enterobacteriaceae: Microbiological Properties
Gram-negative and rod shaped (bacilli) Ferment rather than oxidize D-glucose with acid and (often) gas production Reduce nitrate to nitrite Grow readily on 5% sheep blood or chocolate agar in carbon dioxide or ambient air Grow anaerobically (facultative anaerobes)

13 Enterobacteriaceae: Microbiological Properties
Catalase positive and cytochrome oxidase negative Grow readily on MacConkey (MAC) and eosin methylene blue (EMB) agars Grow readily at 35oC except Yersinia (25o-30oC) Motile by peritrichous flagella except Shigella and Klebsiella which are non-motile Do not form spores

14 Enterobacteriaceae: Natural Habitats
Environmental sites (soil, water, and plants) Intestines of humans and animals

15 Enterobacteriaceae: Modes of Infection
Contaminated food and water (Salmonella spp., Shigella spp., Yersinia enterocolitica, Escherichia coli O157:H7) Endogenous (urinary tract infection, primary bacterial peritonitis, abdominal abscess) Abnormal host colonization (nosocomial pneumonia) Transfer between debilitated patients Insect (flea) vector (unique for Yersinia pestis)

16 Enterobacteriaceae: Types of Infectious Disease
Intestinal (diarrheal) infection Extraintestinal infection Urinary tract (primarily cystitis) Respiratory (nosocomial pneumonia) Wound (surgical wound infection) Bloodstream (gram-negative bacteremia) Central nervous system (neonatal meningitis)

17 Enterobacteriaceae: Urinary Tract Infection, Pneumonia
Urinary tract infection: Escherichia coli, Klebsiella pneumoniae, Enterobacter spp., and Proteus mirabilis Pneumonia: Enterobacter spp., Klebsiella pneumoniae, Escherichia coli, and Proteus mirabilis

18 Enterobacteriaceae: Wound Infection, Bacteremia
Wound Infection: Escherichia coli, Enterobacter spp., Klebsiella pneumoniae, and Proteus mirabilis Bacteremia: Escherichia coli, Enterobacter spp., Klebsiella pneumoniae, and Proteus mirabilis

19 Enterobacteriaceae: Nosocomial Infections in the United States 1986-1989 and 1990-19961
Escherichia coli 27,871 (13.7%) Enterobacter spp. 12,757 (6.2%) Klebsiella pneumoniae 11,015 (5.4%) Proteus mirabilis 4,662 (2.3%) Serratia marcescens 3,010 (1.5%) Citrobacter spp ,912 (1.4%) 1Enteric Reference Laboratory, Centers for Disease Control and Prevention

20 Enterobacteriaceae: Intestinal Infection
Shigella sonnei (serogroup D) Salmonella serotype Enteritidis Salmonella serotype Typhimurium Shigella flexneri (serogroup B) Escherichia coli O157:H7 Yersinia enterocolitica

21 Triple Sugar Iron (TSI) Agar
Yeast extract 0.3% (% = grams/100 mL) Beef extract 0.3% Peptone 1.5% Proteose peptone 0.5% Total Protein = 2.6% Lactose % Sucrose1 1.0% Glucose 0.1% Carbohydrate = 2.1% 1Absent in Kligler Iron Agar

22 Triple Sugar Iron (TSI) Agar
Ferrous sulfate Sodium thiosulfate Sodium chloride Agar (1.2%) Phenol red pH = 7.4

23 TSI Reactions of the Enterobacteriaceae
Yellow deep, purple slant: acid deep due to glucose fermentation , no lactose or sucrose fermentation with alkaline slant due to production of amine’s from protein Black deep, purple slant: acid deep due to glucose fermentation with H2S production, no lactose or sucrose fermentation Yellow deep and slant: acid deep and slant due to glucose as well as lactose and/or sucrose fermentation Black deep and yellow or black slant: acid deep and slant with glucose and lactose and/or sucrose fermentation with H2S production Fracturing or lifting of agar from base of culture tube: CO2 production


25 TSI Reactions of the Enterobacteriaceae
A/A + g = acid/acid plus gas (CO2) A/A = acid/acid A/A + g, H2S = acid/acid plus gas, H2S Alk/A = alkaline/acid Alk/A + g = alkaline/acid plus gas Alk/A + g, H2S = alkaline/acid plus gas, H2S Alk/A + g, H2S (w) = alkaline/acid plus gas, H2S (weak)

26 A/A + g Escherichia coli Klebsiella pneumoniae Klebsiella oxytoca
Enterobacter aerogenes Enterobacter cloacae Serratia marcescens1, 2 1Non-lactose, sucrose fermenter 255% + g

27 A/A Serratia marcescens1, 2 Yersinia enterocolitica2 145% of strains
2Non-lactose, sucrose fermenter

28 A/A + g, H2S Citrobacter freundii Proteus vulgaris1
1Non-lactose, sucrose fermenter

29 Alk/A Shigella Providencia

30 Alk/A + g Salmonella serotype Paratyphi A

31 Alk/A + g, H2S Salmonella (most serotypes) Proteus mirabilis
Edwardsiella tarda

32 Alk/A + g, H2S (w) Salmonella serotype Typhi

33 MacConkey (MAC) Agar Peptone 1.7% Polypeptone 0.3% Lactose1 1.0%
Bile salts % Crystal violet2 Neutral red3 Sodium chloride 0.5% Agar % pH=7.1 1Differential medium for lactose fermentation 2Inhibit gram positives and fastidious gram-negatives; MAC agar selective for gram-negatives 3Red color at pH < 6.8



36 Eosin Methylene Blue (EMB) Agar (Levine)
Peptone 1.0% Lactose1 0.5% Eosin y2 Methylene blue2 Agar pH = 7.2 1Modified formula also contains sucrose (0.5%) 2Inhibit gram-positives and fastidious gram-negatives; selective for gram-negatives. Eosin y and methylene blue form a precipitate at acid pH; differential for lactose fermentation



39 Bacterial Utilization of Lactose
Presence of β-galactoside permease: Transport of β-galactoside (lactose) across the bacterial cell wall Presence of β-galactosidase: Hydrolysis of β-galactoside bond (lactoseglucose + galactose) ONPG: Orthonitrophenyl-β-D-galacto- pyranoside

40 Differential Reactions of the Enterobacteriaceae by TSI, ONPG, and MAC
Escherichia coli Red colonies, (A/A, ONPG+) pitted Klebsiella Red colonies, (A/A, ONPG+) mucoid Enterobacter Red colonies (A/A, ONPG+) Citrobacter Red or colorless (A/A or Alk/A, ONPG+) colonies Serratia Colorless colonies 1K. pneumoniae, indole –, K. oxytoca, indole + 2C. freundii, indole – and H2S +, C. koseri, indole + and H2S –

41 Differential Reactions of the Enterobacteriaceae by TSI, ONPG, and MAC
Shigella Colorless Colonies (Alk/A; ONPG – A, B, and C1; ONPG + D1) Salmonella Colorless Colonies (Alk/A + H2S; ONPG –) Proteus Colorless Colonies (Alk/A + H2S2; ONPG –) Edwardsiella tarda Colorless Colonies (Alk/A + H2S; ONPG–) Yersinia Colorless Colonies (A/A, ONPG +) 1Shigella A, B, and C, ornithine –; Shigella D, ornithine + 2Proteus mirabilis. P. vulgaris sucrose + with A/A + H2S on TSI

42 Differential Reactions of the Enterobacteriaceae by EMB
Escherichia coli Colonies with metallic green sheen Klebsiella Colonies with precipitate (ppt) and mucoid appearance Enterobacter Colonies with ppt Citrobacter Colonies with/without ppt Serratia Colonies without ppt Shigella Colonies without ppt Salmonella Colonies without ppt Proteus Colonies without ppt Yersinia Colonies without ppt

43 ONPG Reaction and Lactose Fermentation (Lac)
ONPG Lac Escherichia coli Shigella sonnei – Citrobacter /– Yersinia enterocolitica – Klebsiella Serratia marcescens –

44 Xylose Lysine Deoxycholate (XLD) Agar: Composition
Lactose % Sucrose % Sodium chloride % Yeast extract % Sodium deoxycholate % Sodium thiosulfate Ferric ammonium citrate Agar % Phenol red pH = 7.4

45 XLD Agar: Growth of Salmonella
Salmonella selective due to bile salt. Xylose fermentation (except Salmonella serotype Paratyphi A) acidifies agar activating lysine decarboxylase. With xylose depletion fermentation ceases, and colonies of Salmonella (except S. Paratyphi A) alkalinize the agar due to amines from lysine decarboxylation. Xylose fermentation provides H+ for H2S production (except S. Paratyphi A).

46 XLD Agar: Appearance of Salmonella
Ferric ammonium citrate present in XLD agar reacts with H2S gas and forms black precipitates within colonies of Salmonella. Agar becomes red-purple due to alkaline pH produced by amines. Back colonies growing on red-purple agar-presumptive for Salmonella.



49 XLD Agar: Growth of Escherichia coli and Klebsiella pneumoniae
Escherichia coli and Klebsiella pneumoniae are lysine-positive coliforms that are also lactose and sucrose fermenters. The high lactose and sucrose concentrations result in strong acid production, which quenches amines produced by lysine decarboxylation. Colonies and agar appear bright yellow. Neither Escherichia coli nor Klebsiella pneumoniae produce H2S.

50 XLD Agar: Growth of Shigella and Proteus
Shigella species do not ferment xylose, lactose, and sucrose, do not decarboxylate lysine, and do not produce H2S. Colonies appear colorless. Proteus mirabilis ferments xylose, and thereby provides H+ for H2S production. Colonies appear black on an agar unchanged in color (Proteus deaminates rather than decarboxylates amino acids). Proteus vulgaris ferments sucrose, and colonies appear black on a yellow agar.



53 Hektoen Enteric (HE) Agar: Composition
Peptone % Yeast extract % Bile salts % Lactose % Sucrose % Salicin % Sodium chloride % Ferric ammonium citrate Acid fuchsin Thymol blue Agar % pH = 7.6

54 HE Agar: Growth of Enteric Pathogens and Commensals
High bile salt concentration inhibits growth of gram-positive and gram-negative intestinal commensals, and thereby selects for pathogenic Salmonella (bile-resistant growth) present in fecal specimens. Salmonella species as non-lactose and non-sucrose fermenters that produce H2S form colorless colonies with black centers. Shigella species (non-lactose and non-sucrose fermenters, no H2S production) form colorless colonies. Lactose and sucrose fermenters (E. coli, K. pneumoniae) form orange to yellow colonies due to acid production.


56 Salmonella-Shigella Agar
Beef extract % Peptone % Bile salts % Sodium citrate % Brilliant green dye Trace Lactose % Sodium thiosulfate % Ferric citrate % Neutral red Agar % pH = 7.4

57 Salmonella-Shigella (SS) Agar
Bile salts, citrates, and brilliant green dye inhibit gram-positives and most gram-negative coliforms Lactose the sole carbohydrate Sodium thiosulfate a source of sulfur for H2S production Salmonella forms transparent colonies with black centers Shigella forms transparent colonies without blackening Lactose fermentative Enterobacteriaceae produce pink to red colonies with bile precipitate for strong lactose fermenters

58 Use of Selective-Differential Agars for Recovery of the Enterobacteriaceae from Different Types of Specimens Feces1: MAC or EMB + XLD &/or SS or HE2 Sputum and Urine1: MAC or EMB Wound3:MAC or EMB Peritoneal and pleural fluid4: MAC or EMB Subculture of blood positive for gram-negative’s in broth culture4: MAC or EMB CSF, pericardial fluid, synovial fluid, bone marrow5: Not required 1Heavy population of commensal bacteria 2Utilized with enrichment broth containing selenite or mannitol to differentially inhibit enteric commensals 3Commensal bacteria (skin) and frequent polymicrobial etiology 4Possible polymicrobial etiology (normally sterile fluids) 5Normally sterile, unimicrobial etiology predominant

59 Selectivity of Differential Agars for Salmonella1 and Shigella2
HE or SS agar (absence of lactose fermentation1,2, H2S production1) XLD agar (absence of lactose fermentation1,2, H2S production1, lysine decarboxylation1) MAC or EMB agar (absence of lactose fermentation1,2) TSI agar (glucose fermentation1,2, absence of lactose fermentation1,2, H2S production1) Descending Order of Selectivity for Salmonella and Shigella

60 Recommended Reading Winn, W., Jr., Allen, S., Janda, W., Koneman, E.,
Procop, G., Schrenckenberger, P., Woods, G. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology, Sixth Edition, Lippincott Williams & Wilkins, 2006: Chapter 5. Medical Bacteriology: Taxonomy, Morphology, Physiology, and Virulence. Chapter 6. The Enterobacteriaceae.

61 Recommended Reading Murray, P., Baron, E., Jorgensen, J., Landry,
M., Pfaller, M. Manual of Clinical Microbiology, 9th Edition, ASM Press, 2007: Farmer, J.J., III, Boatwright, K.D., and Janda J.M. Chapter 42. Enterobacteriaceae: Introduction and Identification

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