Gastrointestinal Infections & Food Poisoning

MLAB 2434 –Microbiology Keri Brophy-Martinez

General Concepts A complete history should be taken

Foods eaten recently Exposure to ill patients Recent travel Medicinal history Any underlying illnesses

Diarrhea may be caused by viruses, bacteria, parasites, food poisoning and non-infectious processes

Usually acquired by ingesting contaminated food or beverage

Anatomy of GI Tract Organisms must be able to survive

gastric acids in order to reach the small bowel

In small bowel, motility (peristalsis) is major host defense. Organisms can not adhere to intestinal wall

Generally, a large dose of organisms is needed to cause disease

Normal GI Flora Stomach contains few organisms Upper part of small bowel contains

small numbers of Enterococcus sp, lactobacilli, and diphtheroids, along with Candida albicans in 20-40% of individuals

Colon contains large numbers of anaerobes and facultative aerobes in 1000:1 ratio Colon produces IgA Pathogens would have to compete with

normal flora

Risk Factors Number of Ingested Organisms

Median infectious dose (ID50)• The number of ingested organisms that

must be ingested to cause a diarrheal illness in 50% of exposed individuals

AchlorhydriaInadequate stomach acidity

Reduction in normal floraUse of antibiotics

HistoryTravel to endemic areas of worldRecreational activitiesExposure to ill patients

FoodDetailed history of food eaten 3

days prior to onset of symptoms

Diagnosing Cause of Diarrhea

Diagnosing Cause of Diarrhea Physical Exam

Dehydration Toxic megacolon Increase in heart rate or decrease in blood

pressure after standing upright

Laboratory Diagnosis CBC Fecal WBC Stool Culture O &P Electrolyte panel

Enterotoxin-Mediated Diarrhea Symptoms

Rapid onset of diarrhea• Less than 12 hours

Lack feverAbsence of blood or pus

• Point to enterotoxin mediated illnessLarge number of watery stools

• Sometimes >20 per day

Enterotoxin-Mediated Diarrhea Pathogens

ETECV. choleraeS. aureusC. perfringensB. cereus

Other typesViral and parasitic can be similar but

symptoms longer lasting

Invasive Diarrhea Invasion of the bowel mucosal surface Symptoms

Fecal leukocytes RBCs Sometimes fever

Organisms Salmonella spp. Campylobacter spp. Shigella spp. E. coli E. histolitica

Common Bacterial Agents Causing Diarrhea

Common Bacterial Agents Causing Diarrhea Campylobacter jejuni

Most common cause of bacterial diarrhea in the world

Inadequately cooked poultry, untreated water, unpasteurized milk, and exposure to animals with diarrhea

Self-limiting, antibiotics not needed Manifests with fever, diarrhea and

abdominal cramping

Campylobacter jejuni:Fast facts Grows best at 42 degrees C Microaerophilic conditions Capnophilic conditions Campy plate Gram-negative curved rods,

“seagull wings”

Common Bacterial Agents Causing Diarrhea (cont’d) Salmonella species

Gastroenteritis and Food Poisoning• Contracted by eating undercooked

meat, poultry, eggs, and contaminated dairy products

• Nausea, vomiting, and diarrhea 6-48 hours after ingestion

• Usually self-limiting; antibiotics discouraged because they can induce “carrier state”

Common Bacterial Agents Causing Diarrhea (cont’d) Salmonella species (cont’d)

Enteric Fever• Typhoid fever is most severe• Contaminated food and water• Organisms invade small bowel & colonic

tissue• Live and reproduce in monocytes• Can invade gall bladder and produce

“carrier state”• Symptoms include headache, fever, malaise

and abdominal tenderness

Salmonella speciesFast Facts Requires a high microbial load for infection Culture

Look for LN on MAC Look for blue green colonies with blk

centers on HE Look for red colonies with blk centers on

XLD Fecal wbc lab test

See rbc’s and wbc’s in stool

Salmonella

Common Bacterial Agents Causing Diarrhea (cont’d) Shigella species

Diarrhea may have blood and pus Symptoms appear 12-50 hours after

exposure Most communicable of the diarrheal

bacteria Symptoms include: fever, malaise,

fatigue and anorexia

Shigella speciesFast Facts Requires a low microbial load Fecal WBC lab test

Observe blood, WBCs, pus Culture

Colorless colonies(NLF) on MACBlue green colonies of HERed/colorless on XLD

Shigella

Common Bacterial Agents Causing Diarrhea (cont’d) Diarrheogenic Escherichia coli

ETEC: Enterotoxigenic• Cause of traveler’s diarrhea

EIEC: Enteroinvasive EPEC: Enteropathogenic

• Diarrhea outbreaks in infants in hospital setting EHEC: Enterohemorrhagic

• E. coli 0157:H7• Presence of shiga-like toxin• Associated with HUS

EAEC: Enteroaggregative• Chronic diarrhea in HIV patients, travelers, &

children in poor countries

E. coli O157:H7 on MAC and SMAC

Common Bacterial Agents Causing Diarrhea (cont’d) Vibrio species

Requires a large microbial load Utilize TCBS media

• Inhibits colonic flora• Differentiates sucrose fermenters from

species of Vibrio that are non-fermenters Stool contains no rbc’s or wbc’s since

it is toxin mediated & non-inflammatory

Common Bacterial Agents Causing Diarrhea (cont’d) Clostridium difficle

Test for the toxin, culture not performed Hospital-acquired due to alteration of

normal flora and use of antibiotics Consists of Toxin A & B

Less Common Agents of GI Illness Yersinia enterocolita Plesiomonas shigelloides Aeromonas hydrophilia Listeria monocytogenes

Common Bacterial Agents Causing Diarrhea (cont’d) Many cases of food poisoning

caused by toxins produced by bacteriaBacteria may no longer be alive,

but toxins can cause food poisoning

Example: S. aureus, Clostridium botulinum, Bacillus cereus

Bacterial Agents Associated with Food Poisoning

Agent Source of Contamination

Toxin Implicated Foods

S. aureus Nasal passages of asymptomatic carriers

Staphylococcal enterotoxin

Foods with mayonnaise, eggs, or dairy products, canned food, frozen food, processed meats

Clostridium botulinum

Soil & water Botulism neurotoxin

Mushrooms, salami, improperly canned foods

Bacillus cereus Environmental contaminant

Heat-stable enterotoxin & heat-labile enterotoxin

Grains, especially rice

Common Parasitic Agents Causing Diarrhea Parasitic Infections

Giardia lamblia• Ingestion of contaminated water or person-person

spread• Nausea, vomiting, flatulence, cramping and diarrhea• Absence of fever and fecal leukocytes

Entamoeba histolytica• Fever, grossly bloody diarrhea

Other rare parasites include:• Cryptosporidium• Cyclospora• Microsporidia• Ascaris• Stronglyloides• Trichuris• And many more…

Common Viral Agents Causing Diarrhea Hard to diagnosis due to virus

size Require secondary testing such

as cell culture, PCR, EM

Common Viral Agents Causing Diarrhea Rotavirus

Primarily affects children < 5 years old

Spread by fecal-oral routePeak incidence from December

through June

Common Viral Agents Causing Diarrhea Calicivirus

Norovirus• Spread by fecal-oral route by

contaminated food or water or environmental fomites, person to person

• Outbreaks on cruise ships• Cause of stomach flu

Laboratory Diagnosis of Gastrointestinal Pathogens Specimen Collection and Handling

Collect with 4 days of onset of symptoms

Stool should be processed ASAP; NOT refrigerated

Rectal swabs NOT recommendedUse of preservatives not

recommended unless test ordered is an O & P

Laboratory Diagnosis of Gastrointestinal Pathogens

Fecal Leukocytes• Direct microscopic exam almost

exclusively performed to detect presence of WBCs and RBCs. Their presence is due to intestinal wall bleeding

• Differentiates invasive disease from toxin-mediated illnesses, viral illness and parasitic infections

• + WBC: Salmonella, Shigella, Yersinia, EIEC, Campylobacter, Vibrio

Positive Fecal WBC

Interpretation of Cultures All media plates work together to

determine the presence of a pathogen. Routine stool cultures include testing

for:Salmonella species, Shigella species,

Aeromonas species, Plesiomonas species, and Campylobacter jejuni

Less frequently orderedYersinia, Vibrio, E. coli 0157:H7

Identification & Reporting of Isolates Presumptive identification tests

Oxidase TSI Urea

Reporting If no pathogens found

• Report “No Salmonella, Shigella, or Campylobacter isolated”

Pathogen Isolated• Identification & quantification• Any amount significant• Additional testing may be indicated

Treatment of Diarrhea Patients must be watched for dehydration Antibiotics are NOT effective against viral

pathogens, give supportive care for hydration Antibiotics may shorten illness by invasive

bacteria or an enterotoxin-mediated process Antidiarrheal mediations (Lomotil, Pepto-Bismol)

Primarily used with enterotoxin mediated diarrhea or viral gastroenteritis

Prophylactic therapy not recommended for travelers

When traveling, “Boil it, peel it, cook it, or forget it”!

References Engelkirk, P., & Duben-Engelkirk, J. (2008). Laboratory

Diagnosis of Infectious Diseases: Essentials of Diagnostic Microbiology . Baltimore, MD: Lippincott Williams and Wilkins.

http://www2.cfpc.ca/cfp/2004/Nov/vol50-nov-cme-1.asp?stype=advanced&

http://www.cdc.gov/rotavirus/index.html Kiser, K. M., Payne, W. C., & Taff, T. A. (2011). Clinical

Laboratory Microbiology: A Practical Approach . Upper Saddle River, NJ: Pearson Education.

Mahon, C. R., Lehman, D. C., & Manuselis, G. (2011). Textbook of Diagnostic Microbiology (4th ed.). Maryland Heights, MO: Saunders.