After returning from a trip to Mexico, which organism is most likely responsible for watery diarrhea in a patient with Lac- colonies?

The organism most likely responsible for watery diarrhea in a patient presenting with Lac- colonies, particularly after returning from a trip to Mexico, is Salmonella typhi. Salmoella typhi is known to cause typhoid fever, which can lead to gastrointestinal symptoms such as diarrhea. Lac- (lactose-negative) colonies indicate that the organism does not ferment lactose, a characteristic that aligns with Salmonella species.

Travel to endemic areas, such as Mexico, can expose individuals to contaminated food or water, increasing the risk of infections caused by enteric pathogens, including Salmonella. Some strains of Salmonella can cause watery diarrhea as part of the infection context, particularly when there is a significant enteric component.

While other organisms on the list can cause diarrhea, they don't align with the Lac- colony characteristic as clearly as Salmonella typhi does, nor are they as strongly associated with travel exposure. Enterohemorrhagic Escherichia coli typically causes bloody diarrhea rather than watery diarrhea and is lactose fermenting (Lac+) in many strains. Shigella dysenteriae and Campylobacter jejuni are also associated with diarrhea, but typically they present differently in culture and symptomatology, and they do not typically yield Lac-