The document outlines gastrointestinal infections, focusing on diarrheal diseases, their causes, and underlying mechanisms. It details various pathogens involved in acute diarrhea, their classifications, and the significance of resident microbial flora. Additionally, it discusses diagnosis methods, treatment approaches, and the prevalence of these infections globally.

1. Gastrointestinal
(GI) Infections

2. Learning objectives
2
At the end of the session, the students will be able to understand:
▰ Types of diarrhoeal diseases.
▰ Infections agents of acute diarrhea and the underlying
mechanism.
▰ Identification features/ detection methods of common
organisms causing acute diarrhea or dysentery and their
pathogenic mechanisms

3. RESIDENT
MICROBIAL FLORA 3

4. RESIDENT MICROBIAL FLORA
4
▰ Human GIT is colonized by diverse group of normal resident
microbial flora.
▰ Upper GIT contains only sparse flora (streptococci in oral cavity
and lactobacilli in stomach).
▰ Lower GIT: The microbial load gradually increases towards
lower part of GIT and is highest in the distal ileum (1011
to
1012
/g).

5. RESIDENT MICROBIAL FLORA
(Cont..)
5
▰ The large bowel comprises of both anaerobes and aerobes with
a ratio of 1,000:1; with Bacteroides fragilis (most common) and
Enterobacteriaceae such as E. coli, Klebsiella present
predominantly
▰ Normal flora also comprises of yeasts (e.g. Candida) and
parasites (e.g. Entamoeba coli).

6. DIARRHEAL
DISEASES 6

7. DIARRHEAL DISEASES
7
▰ Diarrheal diseases are one of the leading cause of illness
globally; cause significant morbidity and mortality.
▰ Worldwide - 1.7 to 5 billion cases of diarrhea occur per year, with
1.26 million deaths; accounting for the second leading cause of
death globally

8. Gastrointestinal infective
syndromes
8
Diarrheal diseases
 Diarrhea
 Dysentery
 Traveler’s diarrhea
 Persistent (chronic) diarrhea
 Gastroenteritis
 Food poisoning
Other gastrointestinal infective syndrome
 Acute vomiting
 Necrotizing enterocolitis
 Necrotizing enteritis
 Pseudomembranous enterocolitis
 Tropical sprue
 Peptic ulcer disease
 Infections of other GI structures such as appendicitis, diverticulitis

9. Diarrhea
9
▰ Diarrhea - defined as passage of three or more loose or liquid
stools per day, in excess than the usual habit for that person
(World Health Organization).
▰ Caused by microbial infections, or as a result of other
gastrointestinal diseases such as inflammatory bowel diseases,
coeliac disease, etc.
▰ Acute diarrhea usually lasts for <14 days; most often caused by

10. Dysentery
10
▰ Dysentery is characterized by diarrhea with increased blood and
mucus, often associated with fever, abdominal pain, and
tenesmus (feeling of constant need to pass stools, despite an
empty colon)

11. Infections agents of acute diarrhea and
the underlying mechanism
11
Mechanism Features Examples of pathogens involved
Non-inflammatory Location:
Proximal small bowel
Illness:
Watery diarrhea
Stool findings:
 No fecal leukocytes
 Fecal lactoferrin—
not increased or
mildly increased
Bacteria (mostly
enterotoxin mediated):
 Vibrio cholerae
 Escherichia coli:
 Enteropathogenic
(nontoxigenic)
 Enterotoxigenic
 Enteroaggregative
 Clostridium
perfringens
 Bacillus cereus
 Staphylococcus
aureus

Parasites (Protozoa):
 Giardia lamblia
 Cryptosporidium
parvum
 Cyclospora
cayetanensis
 Cystoisospora belli
 Blastocystis hominis

12. Infections agents of acute diarrhea and
the underlying mechanism (Cont..)
12
Mechanism Features Examples of pathogens involved
Non-inflammatory Location:
Proximal small bowel
Illness:
Watery diarrhea
Stool findings:
 No fecal leukocytes
 Fecal lactoferrin—
not increased or
mildly increased
Viruses:
 Rotavirus
 Norovirus
 Enteric
adenoviruses- 40,41
 Caliciviruses
 Astrovirus
Fungi:
Microsporidia
Parasites (Helminths):
 Ascaris lumbricoides
 Hookworm
 Strongyloides
stercoralis
 Enterobius
vermicularis
 Trichinella spiralis
 Taenia saginata, T.
solium
 Hymenolepis nana
 Dipylidium caninum
 Diphyllobothrium

13. Infections agents of acute diarrhea and
the underlying mechanism (Cont..)
13
Mechanism Features Examples of pathogens involved
Inflammatory
(invasion or
cytotoxin)
Location:
Colon or distal small
bowel
Illness:
 Dysentery or
 Inflammatory
diarrhea
Stool findings:
 Fecal pus cells
(polymorphonuclear
leukocytes)—
increased
 Fecal lactoferrin—
markedly increased
Predominantly dysentery:
 Shigella species
 Campylobacter jejuni
 Enterohemorrhagic E. coli
 Enteroinvasive E. coli
 Vibrio parahaemolyticus
Predominantly
inflammatory diarrhea:
 Non-typhoidal
salmonellae
 Yersinia enterocolitica
 Listeria monocytogenes
 Clostridioides difficile
 Plesiomonas shigelloides
Parasite:
(predominantly
dysentery)
 Entamoeba histolytica
 Balantidium coli
 Trichuris trichiura
 Schistosoma mansoni
 Schistosoma
japonicum

14. Infections agents of acute diarrhea and
the underlying mechanism (Cont..)
14
Mechanism Features Examples of pathogens involved
Penetrating Location: Distal small
bowel
Illness: Enteric fever
Stool findings:
Fecal mononuclear
leukocytes ( )
↑
Salmonella Typhi, (enteric fever)
Yersinia enterocolitica (typhoid like illness)

15. Traveler’s Diarrhea
15
▰ Most common travel-related infectious illness.
▰ Clinical presentation: Most cases begin within the first 3–5
days; characterized by a sudden onset of abdominal cramps,
anorexia, and watery diarrhea.
▰ The illness is generally self-limited, lasting for 1–5 days

16. Traveler’s Diarrhea (Cont..)
16
▰ Overall, enterotoxigenic Escherichia coli is the most common
agent, followed by enteroaggregative E. coli.
▰ Campylobacter jejuni - more common in Asia.
▰ Norovirus diarrhea - associated with traveling on cruise ships.

17. Agents causing traveler’s diarrhea
17
Etiologic agent Comments
Bacteria (50–75%)
Enterotoxigenic E. coli (10–45%) Single most important agent
Enteroaggregative E. coli (5–35%) Emerging enteric pathogen with
worldwide distribution
Campylobacter jejuni (5–25%) More common in Asia
Shigella, Enteroinvasive E.coli Major cause of dysentery
Non-typhoidal salmonellae Common agent in India
Others Including Aeromonas,
Plesiomonas, and Vibrio cholerae

18. Agents causing traveler’s diarrhea
(Cont..)
18
Etiologic agent Comments
Viruses (<20%)
Norovirus (<10%) Associated with cruise ships
Rotavirus (<5%) Common among children
Parasites (0–10%) Giardia lamblia, Cryptosporidium,
Entamoeba histolytica, Cyclospora

19. Persistent and Chronic Diarrhea
19
▰ Diarrhea that lasts for 14 days (usually 2-4 weeks) is
≥
considered persistent.
▰ Chronic diarrhea usually lasts for >4 weeks.
▰ May result from infections due to various organisms.
▰ Parasites (e.g. Cryptosporidium, Cyclospora, Entamoeba histolytica,
Giardia) account for a major cause of chronic diarrhea

20. Persistent and Chronic Diarrhea
(Cont..)
20
▰ Bacteria (e.g. Aeromonas, Campylobacter, Clostridium difficile,
Plesiomonas)
▰ Viruses - cytomegalovirus, common in immunocompromised
host
▰ Fungi - microsporidia, common in immunocompromised host.

21. Gastroenteritis
21
▰ Gastroenteritis or infectious diarrhea - defined as inflammation
of the mucous membrane of the stomach and intestine resulting
in combination of diarrhea, vomiting and pain abdomen with or
without mucus or blood in stool, fever or dehydration.

22. Food Poisoning
22
▰ Food poisoning refers to an illness acquired through
consumption of food or drink contaminated either with
microorganisms, or their toxins

23. Pathogenic Mechanisms
23
▰ Inoculum Size
▰ Adherence
▰ Toxin Production
▰ Invasion

24. Pathogenic mechanisms of diarrheal
agents
24
Toxin production
Enterotoxins
Cholera toxin
Vibrio parahaemolyticus
E. coli
 LT and ST of ETEC
 EAST of EAEC
 VT of EHEC
Clostridioides difficile (toxin A)
Aeromonas
Rotavirus (NSP4)
Campylobacter jejuni
Cytotoxins
 Shigella dysenteriae type 1
 Enterohemorrhagic E. coli
 Clostridioides difficile (toxin B)
Neurotoxins
 Staphylococcus aureus
enterotoxin
 Bacillus cereus toxin
 Clostridium botulinum toxin

25. Pathogenic mechanisms of diarrheal
agents (Cont..)
25
Attachment within
or
close to mucosal
cells
Invasion of intestinal epithelium
E. coli
 Enteropathogenic
 Enterohemorrhagi
c
Cryptosporidium
species
Cyclospora species
Cystoisospora species
Rotavirus
Norovirus
Shigella species
Enteroinvasive E. coli
Campylobacter jejuni
Yersinia enterocolitica
Plesiomonas shigelloides
Entamoeba histolytica
Balantidium coli

26. Predisposing Factors
26
▰ Suppression of the normal flora
▰ Neutralization of gastric acidity
▰ Inhibition of intestinal motility
▰ Age and location
▰ Antibiotic-associated
▰ Impaired host immunity
▰ Genetic determinants

27. Laboratory Diagnosis
27
Specimen Collection:
▰ Fecal specimen (containing mucus flakes) - collected in a sterile
screw capped wide mouthed container.
▰ In carriers - a rectal swab may be collected.
▰ Specimens should be transported to the laboratory within 1
hour
▰ If a delay of longer than 1 hour is anticipated - fecal specimen
should be collected in transport media

28. Laboratory Diagnosis (Cont..)
28
Macroscopy:
▰ Color of the specimen
▰ Consistency of the specimen—formed, semiformed or liquid
▰ Presence of blood (suggestive of dysentery), mucus or pus
(suggestive of inflammatory diarrhea)
▰ Presence of adult parasitic forms, e.g. Enterobius, Ascaris, or
Taenia segments.

29. Laboratory Diagnosis (Cont..)
29
Microscopy:
▰ Wet mount preparation
▰ Hanging drop preparation
▰ Gram stained smear
▰ Modified acid fast staining
▰ Electron microscopy

30. Laboratory Diagnosis (Cont..)
30
Bacterial Culture:
▰ Culture media
▰ Identification
▰ Antimicrobial susceptibility test

31. Laboratory Diagnosis (Cont..)
31
Tissue Culture:
▰ This is carried out for the detection of enteric viruses and also
for some diarrheagenic E. coli.
▰ Enterotoxigenic Escherichia coli (ETEC) penetrates HeLa and HEp-
2 cell line, whereas verocytotoxin of enterohemorrhagic
Escherichia coli (EHEC) is detected by its cytotoxic effect on Vero
cell line.

32. Laboratory Diagnosis (Cont..)
32
Antigen Detection:
▰ ELISA is available for detection of rotavirus antigen in Stool
▰ Rapid tests - immunochromatographic test is available for
simultaneous detection of antigens of three parasites
Entamoeba histolytica, Giardia and Cryptosporidium in stool
▰ Latex agglutination test - available to detect E. coli O157 antigen
in stool
▰ Rapid test - available to detect C. difficile antigens (glutamate
dehydrogenase and toxin A/B) in stool.

33. Laboratory Diagnosis (Cont..)
33
Molecular Methods:
▰ Polymerase chain reaction (PCR) assays can be carried out for
the detection specific genes of enteric pathogens
▰ BioFire FilmArray: Fully automated commercial nested
multiplex PCR. Its gastrointestinal panel is used to detect
common bacterial, viral, parasitic diarrheal pathogens.

34. Laboratory Diagnosis (Cont..)
34
Other Methods:
▰ Methylene blue preparation
▰ Fecal lactoferrin

35. Identification features/ detection methods of
common organisms causing acute diarrhea or
dysentery.
35
Enteric
bacteria
Presentation Identification features
Vibrio cholerae Watery diarrhea Darting motility
Coma-shaped gram-negative bacilli in culture
smear TCBS agar: Sucrose fermenting yellow
colored colonies
Identification: By conventional biochemical tests or automated
methods like VITEK or MALDI-TOF Agglutinates with Vibrio cholerae
O1 antisera and ogawa antisera (this is the most common pattern;
though other serotypes are also present)
Shigella Dysentery Gram-negative bacilli, non-motile
MAC or DCA: Non-lactose fermenting
translucent colonies XLD: Red colonies without
black center
Identification: By conventional biochemical tests or automated

36. Identification features/ detection methods of
common organisms causing acute diarrhea or
dysentery (Cont..)
36
Enteric
bacteria
Presentation Identification features
Group B
Salmonella
Inflammatory
diarrhea
Gram-negative bacilli, motile
MAC: Non-lactose fermenting translucent colonies
DCA: Non-lactose fermenting colonies with
black center XLD: Red colonies with black
center
Identification: By conventional biochemical tests or automated
methods like VITEK or MALDI-TOF
Agglutinates with Salmonella poly-O antisera and serotype (O4)
specific antisera
Viral agents Diarrhea Agents: Rotavirus, Norovirus, Adenovirus 40, 41, etc.
 Detection of viral particles in stool specimen by electron
microscopy
 Detection of viral antigen by ELISA or

37. Identification features/ detection methods of
common organisms causing acute diarrhea or
dysentery (Cont..)
37
Intestinal
parasites
Presentation Stool microscopy detects
Entamoeba
histolytica
Dysentery  Trophozoites and/or quadrinucleated round cyst
 Detection of specific antigen (e.g. lectin)/specific genes in stool
Giardia
intestinalis
Fatty diarrhea Trophozoites (tear drop-shaped binucleated) with four
pairs of flagella and/or Tetra-nucleated oval cyst with a
central axoneme
Trichuris Dysentery Barrel-shaped eggs with mucus plugs at both ends, bile stained
Enterobius
vermicularis
Nocturnal anal
pruritus
Plano-concave egg containing larva, nonbile stained
Ascaris
lumbricoides
Malabsorption Fertilized egg: Round-oval, thick albumin coat, floats in saturated
saline, bile stained Unfertilized egg: Elongated, thin albumin coat,
does not float in saturated saline, bile stained
Hookworm Diarrhea, anemia Egg: Oval, contains segmented ovum with four blastomeres, nonbile
stained

38. Treatment of Diarrhea
38
Treatment depends up on the severity.
▰ Fluid therapy (rehydration) is the main stay of treatment
▰ Anti-motility agents and adsorbents may be considered in
moderate-to-severe diarrhea
▰ Empiric antibiotic therapy is required only for severe diarrhea

39. Treatment of diarrheal diseases
(adapted from Sanford guideline).
39
Types of diarrhea Definition Treatment
Mild diarrhea 1–2 unformed stools per
day Minimal symptoms
No interference with daily routine
activities
Hydration: Fluids
Lactose-free diet and avoid caffeine
No antibiotics required
Moderate diarrhea 3–5 unformed stools per
day With/without
symptoms,
Interferes with daily routine activities
Fluids + Anti-motility
agents (loperamide) +
Adsorbents (bismuth
subsalicylate)
No antibiotics required
Severe diarrhea ≥6 unformed stools per day ±
temperature
≥101ºF, tenesmus, blood or fecal
leukocytes
Empirical: Ciprofloxacin or
levofloxacin for 3–5 days
Alternatives:
 Azithromycin (for 3 days) for
Campylobacter

40. OTHER INFECTIVE
SYNDROMES OF GIT
40

41. OTHER INFECTIVE SYNDROMES
OF GIT
41
 1. Acute Nausea and Vomiting
 2. Necrotizing Enterocolitis
 3. Necrotizing Enteritis
 4. Pseudomembranous Enterocolitis
 5. Tropical Sprue

42. OTHER INFECTIVE SYNDROMES
OF GIT (Cont..)
42
 6. Esophagitis
 7. Gastritis
 8. Appendicitis
 9. Diverticulitis
 10. Typhlitis
 11. Whipple’s Disease (Tropheryma whipplei)

43. Questions:
43
▰ Q1. Single most important agent of travelers diarrhea:
a. Enterotoxigenic E. coli
b. Enteroaggregative E. Coli
c. Campylobacter jejuni
d. NTS

44. Questions:
44
▰ Q2. Examples of bacterial pathogens causing inflammatory
acute diarrhoea:
a. Bacillus cereus
b. Enteroaggregative E. Coli
c. Norovirus
d. NTS