What is your diagnosis?
It could be a case of traveler's diarrhea.

What is diarrhea?
Acute diarrhea is defined as the abrupt onset of abnormally high fluid content in the stool. There is also an increased frequency of bowel movements, which can range from 4-5 to more than 20 times per day.

What is traveler's diarrhea?
When people visit regions of the world where standards of hygiene are low and consume food or water that had been contaminated by E.coli, they suffer from diarrheal diseases. This condition is known by other names such as "Turista" or "Delhi belly".

What is the pathogenesis of this condition?
When food or water contaminated by pathogenic E.coli such as enterotoxigenic E. coli is ingested, the bacteria colonize the intestine and produce enterotoxin that cause diarrhea. The infection requires a relatively high infectious dose (10⁶-10⁸). ETEC strains cause diarrhea through the action of the two plasmid encoded enterotoxins, LT and ST. These strains may express an LT only, an ST only, or both an LT and an ST. The illness is typically abrupt in onset with a short incubation period (14 to 50 hours). The diarrhea is watery, usually without blood, mucus, or pus; fever and vomiting are present in a minority of patients. ETEC diarrhea may be mild, brief, and self-limiting or may result in severe purging similar to that seen cholera.

Which are the other organisms that can cause diarrhea?
Bacteria such as  Shigella sps, Salmonella sps, Campylobacter sps, Yersinia sps, Aeromonas sps, Plesiomonas sps, Klebsiella sps, and Clostridium perfringens ; viruses such as Rotavirus, Calicivirus, Astrovirus, and Norovirus; parasites such as Giardia lamblia, Cryptosporidium can cause varying degrees of diarrhea.

What is the specimen collected and what is the role of laboratory diagnosis?
In most adult patients, this condition is mild and self-limiting. Severe diarrhea in any age group can be dangerous. Laboratory diagnosis is usually not required, however if a laboratory diagnosis must be done, feces sample may be collected in a sterile wide mouthed container. Laboratory may help in differentiating travelers' diarrhea from cholera. A hanging drop may be performed to look for any Vibrio like motility. The sample must be inoculated on to routine culture media like MacConkey's agar as well as TCBS Agar to rule out Vibrio cholerae.

What is your observation?
Hanging drop of feces specimen shows no bacteria with darting-type motility. Pink coloured (lactose fermenting), smooth, low convex, circular colonies of a single type is seen on MacConkey's agar. Gram stained smear of the colony shows gram negative bacilli, hanging drop of culture shows motile bacilli, and catalase test is positive. Results of biochemical reactions are positive indole test, negative urea hydrolysis, negative citrate utilization, positive MR test and negative VP test. TSI agar shows acid slant/acid butt with little gas but no H₂S. The isolate is identified as Escherichia coli.

Isolation of E. coli from feces is normal, how do you identify the pathogen?
It is impossible to differentiate pathogenic (ETEC) E. coli colonies from normal fecal E. coli in a culture. ETEC typically belong to following serotypes; O6, O8, O15, O25, O63, O78, O148 and O159. Detection by any of these serotypes in culture by slide agglutination using specific antisera is diagnostic. Several colonies must be tested before ruling out ETEC. These strains are differentiated from nontoxigenic E.coli present in the bowel by a variety of in-vitro immunochemical, tissue culture, or DNA hybridization tests designed to detect either the toxins or genes that encode for these toxins. Detection of LTs is achieved by ligated rabbit ileal loop test, morphological changes in Chinese hamster ovary cells and Y1 adrenal cells, ELISA, immunodiffusion, co-agglutination etc. STs can be detected by Infant mouse assay and ELISA.

Which are the other enterovirulent E. coli and what do they cause?
Other enterovirulent E.coli that cause diarrhoeal diseases are enteroinvasive E.coli (EIEC) causing dysentery like illness, enterohemorrhagic E.coli (EHEC) causing bloody diarrhea, enteropathogenic E.coli (EPEC) causing infantile diarrhea, and enteroaggregative E.coli (EAggEC), which too has been associated with non-bloody diarrhea in children.