Microbiology Lab Quiz 7

What are some characteristics of ALL enteric bacteria?

• Gram (-)

• Rod-shaped

• Ferments glucose to acid

• Facultative anaerobes (continue growth in the presence of O₂

Family of Enterobacteriaceae; led to the establishment of the Total Coliform Rule

Total Coliform Rule

places limits on the # of E. coli that can be present in water that is used for different purposes due to the fact members of the Enterobacteriaceae cause 40-45% of ALL foodborne and waterborne illnesses in the U.S.

Enteric Bacteria

• relating to or occurring in the intestines or intestinal ailments (E. coli, Shigella, Salmonella), but some occur in non-intestinal habitats (Yersinia pestis, Proteus mirabilis, Erwinia)

• most are considered opportunistic pathogens (E. coli & Proteus vulgaris) and in rare cases trigger septic infections

*Responsible for 40-50% of all nosocomial infections

Escherichia coli

normal inhabitant of human intestines (enteric)

Shigella species

fecal contaminated water; causes dysentery (enteric)

Salmonella species

undercooked chicken/eggs; Most common cause of food poisoning (US-enteric)

Yersinia pestis

rodents, fleas; causes bubonic plague (non-intestinal)

Proteus mirabilis

found in soil; degrades organic material (non-intestinal)

Erwinia species

common plant pathogen; negatively impacts pears, apples, carrots, etc. (non-intestinal)

Opportunistic pathogens

cause diseases (urinary tract infections, pneumonia, & wound infections) in immunocompromised (weakened/impaired immune systems) hosts like AIDS pts, cancer pts, transplant pts, and pregnant women

ex. Proteus vulgaris & Escherichia coli (in rare cases can cause septic infections)

Septic infections

• overactive immune response throughout body

• occurs in response to bacterial infection

• can trigger organ failure and drop in blood pressure which can lead to death

Gram stain

Bacteria: E. coli (-) & Salmonella or S. aureus (+)
Primary stain: Crystal violet
Secondary stain: Safranin
Mordant: Iodine
Decolorizer: Acetone alcohol

MacConkey agar

Selective for Gram - bacteria + Differential (b/t lactose-E. coli & non lactose-Salmonella fermenters) must be checked < or at 24 hrs

Selective component: Bile salts + Crystal violet

Differential component: Lactose

pH indicator: Neutral red (turns pink during lactose fermentation…yellow non)

What agar is used in this test? What bacteria was used and what does the reaction suggest?

MacConkey agar, Salmonella, and the reaction suggests that Salmonella is negative for lactose fermentation

What agar is used in this test? What bacteria was used and what does the reaction suggest?

MacConkey agar, E. coli, and the reaction suggest that E. coli is positive for lactose fermentation

Hektoen agar

Selective for Gram + bacteria & Differential (differentiates b/t microbes that can-Salmonella /cannot produce H₂S from sulfur-Shigella)

Selective component: Bile salts

Differential component: Sulfur and Iron

What agar is used in this test? What bacteria was used and what does the reaction suggest?

Hektoen agar, Shigella, and the reaction suggests that Shigella does not produce H₂S (colonies appear green)

What agar is used in this test? What bacteria was used and what does the reaction suggest?

Hektoen agar, Salmonella, and the reaction suggests that Salmonella does produce H₂S from sulfur, which reacts with iron (colonies appear black)

Indole broth

Differential broth (differentiates b/t microbes that does-E. coli / doesn’t-Salmonella produce tryptophanase enzyme)

Differential component: Tryptophan

After 24 hrs of incubation, add 6 drops of Kovac’s reagent to each tube to see reaction.

What medium is used in this test? What bacteria was used and what does the reaction suggest?

Indole broth, E. coli, and the reaction suggests that E. coli is positive for tryptophanase

What medium is used in this test? What bacteria was used and what does the reaction suggest?

Indole broth, Salmonella, and the reaction suggests that Salmonella is negative for tryptophanase

Citrate agar

Selective and Differentiates for microbes that do-Salmonella / don’t-E. coli use citrate as a carbon (food) source

Differential component: Citrate

pH indicator: Bromothymol blue (green at acidic pH & blue at basic pH)

What agar is used in this test? What bacteria was used and what does the reaction suggest?

Citrate agar, Salmonella, and the reaction suggests that Salmonella is positive for citrate utilization as a food source (basic pH)

What agar is used in this test? What bacteria was used and what does the reaction suggest?

Citrate agar, E. coli, and the reaction suggests that E. coli is negative for citrate utilization as a food source (acidic pH)

Overview of Lab 7: