Enteric Bacilli

Question 1

Natural Environment?

Answer 1

Members of the order Enterobacterales are ubiquitous in nature. With the exception of a few
species, most are present in the intestinal tract of animals and humans as commensal flora. Some
species exist as free-living organisms in the soil, water, and sewage, while others are plant
pathogens.
Outside their natural habitat (i.e., normal intestinal flora), these organisms may produce serious
extraintestinal opportunistic infections. For example, E. coli, the most common facultative
member of the normal bowel flora, may cause fatal meningitis in the newborn and very
commonly causes urinary tract infections in persons of any age group. Environmental or
endogenous enterics can cause devastating infections in the compromised host if wounds are
contaminated with soil, water, or fecal-associated body fluids.

Ubiquitous in nature, found in our normal microbiota, in soil, water, sewage, everywhere. A lot of them cause these opportunistic infections in humans and plants. Mostly opportunistic with indigenous (coming from your own body) or exogenous infections. (except for 3 genus’s) (E. Coli is the most frequently isolated opportunistic in this group. (travelers diarrhea is e. coli).

Question 2

Key Characteristics

Answer 2

Gram reaction: Gram-negative

Shape: Rod-shaped (bacilli)

Arrangement: Single or in pairs; no spores

Motility: Many are motile by peritrichous flagella (flagella all around), but some are nonmotile (e.g., Klebsiella)

Oxygen requirements: Facultative anaerobes — can grow with or without (anaerobic) oxygen but prefer having oxygen

Catalase: Positive

Oxidase: Negative (important distinguishing test from Pseudomonas)

Question 3

Endotoxin

Answer 3

Only found in gram negative bacterium, it’s part of the lipopolysaccharides of the outer membrane of the cell wall of these organisms. LPS is made up of the lipid A core polysaccharide and O antigen. Lipa A portion is the endotoxin and this endotoxin is only released when the gram negative is dying. So it’s a last resort measure for this bacteria. The release of this toxin can cause hypotension, shock ,fever, DIC and death.

Don’t confuse with endospore. Endospore is an vegetative state produced by gram positive rod. Whereas endotoxin is made by all gram negative bacteria.

Question 4

Exotoxin

Answer 4

Can be made by both gram negative and gram positive bacterium. These are toxic protein that are released from a living bacteria into the surrounding environment.

Question 5

Enterotoxin

Answer 5

The enterotoxin can be produced in food and ingested or produced in the intestine. This toxin causes diarrhea or vomiting.
Examples:
Salmonella spp. - Many species (serotypes) that cause gastroenteritis produce enterotoxins
that have been implicated as a significant virulence factor.

Shigella dysenteriae - S. dysenteriae produces a potent exotoxin or “shiga toxin” that causes
hemorrhages leading to bloody diarrhea.

Escherichia coli - The enterotoxigenic E. coli can produce both a heat-stable and a heat-
labile enterotoxin. The heat-labile toxin is similar to the cholera toxin.

Enterohemorrhagic E. coli (such as E. coli O157:H7) produces two toxins, one of which is
identical to the shiga toxin of Shigella dysenteriae

Staph. Aureus as well

Is an exotoxin that acts on the intestinal tract. It causes diahhria or vomiting, it’s produced in food and ingested or the bacteria inside our body can produce it and cause problems in the intestine.

Question 6

Antigenic Structure

Answer 6

A. Flagellar or “H” Antigens
B. Capsular or “K” Antigens
C. Somatic or “O” antigens
Antigens are found on the outer membrane of gram negative rods and we test for them in the lab to help differentiate different strengths between species.

Question 7

A. Flagellar or “H” Antigens

Answer 7

From the German “hauch” or film. Weil and Felix used swarming strains of Proteus vulgaris for
antigenic study and equated the swarm or film with motility. The flagellar antigens are protein
in nature and heat-labile

Heat-labile and it’s only present in mole bacteria.

Question 8

B. Capsular or “K” Antigens

Answer 8

Capsules are surface polysaccharides and also heat-labile

Capsules that make a surface polysaccharide and heat-labile. aka K for Capsule

Question 9

C. Somatic or “O” antigens

Answer 9

Hear staple antigen that’s associated with the body or cell wall of the bacteria.

Question 10

Metabolic Characteristics

Answer 10

Ferment glucose with acid and sometimes gas production

Reduce nitrate (NO₃⁻) → nitrite (NO₂⁻)

Do not produce cytochrome oxidase (oxidase-negative)

Many ferment lactose (e.g., E. coli, Enterobacter, Klebsiella), while others do not (Salmonella, Shigella, Proteus

Question 11

Cultural Characteristics

Answer 11

Grow well on routine media (e.g., nutrient agar, blood agar)

Selective/differential media:

MacConkey agar: lactose fermenters → pink colonies; non-lactose fermenters → colorless

EMB agar: E. coli shows metallic green sheen

Question 12

Normal flora:

Answer 12

Inhabit intestinal tracts of humans and animals

Question 13

Primary Pathogens

Answer 13

Primary pathogens: Salmonella spp., Shigella
spp., Shiga-Toxin-Producing E. coli (STEC),
Yersinia spp.

Question 14

Salmonella spp.

Answer 14

Acquire:

Contaminated food and water.

Infections:

Salmonellosis (gastroenteritis): fever, diarrhea, abdominal cramps

Typhoid fever (enteric fever): caused by Salmonella Typhi — systemic infection with prolonged fever, rose spots, enlarged spleen

Salmonella not typhi — causes just acute gastroenteritis can lead to bacteremia

Bacteremia or septicemia (in immunocompromised patients)

Common environment:

Intestinal tracts of animals (especially poultry, reptiles, cattle)

Contaminated food (raw eggs, chicken, meat) and water

Question 15

Shigella spp.

Answer 15

DIRTY — Shigella disinter a it’s the Shiga toxin producing bacteria that causes bloody diahhria
FINGERS — flexinary
BRINGS — boydii
SHIGELLA — sonii

Infections:

Shigellosis (bacillary dysentery):

Bloody, mucoid diarrhea with abdominal cramps and fever

Caused by mucosal invasion and Shiga toxin (in S. dysenteriae)

Common environment:

Humans are the primary or natural reservoir

Spread by fecal–oral route via contaminated hands, food, or water

Question 16

Shiga-Toxin–Producing E. coli (STEC, a.k.a. EHEC)

Answer 16

Characteristics:

Subgroup of Escherichia coli that produces Shiga toxin

Never normal flora

Infections:

Hemorrhagic colitis: bloody diarrhea with little or no fever

Hemolytic uremic syndrome (HUS): renal failure with a low platelet count

Common environment:

Found in intestinal tracts of cattle and other ruminants

Transmitted by undercooked beef, unpasteurized milk/juice, or contaminated produce

Question 17

Yersinia spp.

Answer 17

Characteristics:

Gram negative rods

Gram-negative, bipolar (“safety pin”) staining coccobacilli

Infections:

Y. pseudotuberculosis: causes appendicitis in humans

Enterocolitis (acute enteritis)

Yersinia pestis:

Plague (bubonic, septicemic, pneumonic) aka black death — transmitted by the bite of rat fleas

Common environment:

Zoonotic – found in pigs, rodents, and wild animals

Transmitted via contaminated food (especially pork), water, or animal contact