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Gastrointestinal Infections & Food Poisoning
MLAB 2434 –Microbiology Keri Brophy-Martinez
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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
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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
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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
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Risk Factors Number of Ingested Organisms Achlorhydria
Median infectious dose (ID50) The number of ingested organisms that must be ingested to cause a diarrheal illness in 50% of exposed individuals Achlorhydria Inadequate stomach acidity Reduction in normal flora Use of antibiotics
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Diagnosing Cause of Diarrhea
History Travel to endemic areas of world Recreational activities Exposure to ill patients Food Detailed history of food eaten 3 days prior to onset of symptoms
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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
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Enterotoxin-Mediated Diarrhea
Symptoms Rapid onset of diarrhea Less than 12 hours Lack fever Absence of blood or pus Point to enterotoxin mediated illness Large number of watery stools Sometimes >20 per day
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Enterotoxin-Mediated Diarrhea
Pathogens ETEC V. cholerae S. aureus C. perfringens B. cereus Other types Viral and parasitic can be similar but symptoms longer lasting
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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
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Common Bacterial Agents Causing Diarrhea
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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
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Campylobacter jejuni: Fast facts
Grows best at 42 degrees C Microaerophilic conditions Capnophilic conditions Campy plate Gram-negative curved rods, “seagull wings”
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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”
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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
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Salmonella species Fast 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
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Salmonella
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Common Bacterial Agents Causing Diarrhea (cont’d)
Shigella species Diarrhea may have blood and pus Symptoms appear hours after exposure Most communicable of the diarrheal bacteria Symptoms include: fever, malaise, fatigue and anorexia
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Shigella species Fast Facts
Requires a low microbial load Fecal WBC lab test Observe blood, WBCs, pus Culture Colorless colonies(NLF) on MAC Blue green colonies of HE Red/colorless on XLD
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Shigella
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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
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E. coli O157:H7 on MAC and SMAC
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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
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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
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Less Common Agents of GI Illness
Yersinia enterocolita Plesiomonas shigelloides Aeromonas hydrophilia Listeria monocytogenes
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Common Bacterial Agents Causing Diarrhea (cont’d)
Many cases of food poisoning caused by toxins produced by bacteria Bacteria may no longer be alive, but toxins can cause food poisoning Example: S. aureus, Clostridium botulinum, Bacillus cereus
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Bacterial Agents Associated with Food Poisoning
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
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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…
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Common Viral Agents Causing Diarrhea
Hard to diagnosis due to virus size Require secondary testing such as cell culture, PCR, EM
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Common Viral Agents Causing Diarrhea
Rotavirus Primarily affects children < 5 years old Spread by fecal-oral route Peak incidence from December through June
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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
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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 recommended Use of preservatives not recommended unless test ordered is an O & P
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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
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Positive Fecal WBC
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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 ordered Yersinia, Vibrio, E. coli 0157:H7
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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
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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”!
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References Engelkirk, P., & Duben-Engelkirk, J. (2008). Laboratory Diagnosis of Infectious Diseases: Essentials of Diagnostic Microbiology . Baltimore, MD: Lippincott Williams and Wilkins. 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.
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