Enteric Gram Negative Bacilli: Enteric Pathogens

The organisms in this section are true primary intestinal pathogens: they are not normal gut flora, and their presence in a stool specimen is always considered clinically significant, never a normal or incidental finding.

Morphology. Gram negative bacilli, motile, peritrichous flagella.

Key biochemical features. Lactose nonfermenting, urease negative, H2S positive, produces hydrogen sulphide, giving black colony centres on selective media such as XLD or Hektoen enteric agar, a classic and clinically useful distinguishing feature from Shigella, which is H2S negative.

Reservoir. Broad animal reservoir, poultry, eggs, reptiles, and other livestock. Humans are also the specific reservoir for Salmonella Typhi and Salmonella Paratyphi, the typhoidal serovars, which have no significant animal reservoir, a critical distinction from the nontyphoidal serovars.

Transmission. Faecal-oral, via contaminated food, poultry, eggs, unpasteurised dairy, for nontyphoidal disease. Faecal-oral, typically via contaminated food or water in areas of inadequate sanitation, for typhoidal disease, typhoid or enteric fever, reflecting person-to-person or chronic carrier-to-person spread given the human-restricted reservoir.

Virulence factors. Type III secretion systems inject effector proteins into host intestinal epithelial cells, promoting bacterial uptake and invasion. Intracellular survival within macrophages is particularly relevant to the systemic, typhoidal disease pattern, where the organism disseminates via the reticuloendothelial system rather than remaining confined to the gut lumen.

Disease patterns. Nontyphoidal salmonellosis is self-limited gastroenteritis, watery or occasionally inflammatory diarrhoea, abdominal cramps, fever, typically resolving without antibiotic treatment in an otherwise healthy host.

Antibiotics are generally reserved for severe disease or high-risk patients, extremes of age, immunosuppression, given the risk of prolonging the carrier state without meaningfully shortening illness duration in uncomplicated cases.

It can rarely cause bacteraemia and metastatic focal infection, particularly in immunocompromised patients, with a specific and well-recognised association between nontyphoidal Salmonella bacteraemia and sickle cell disease patients developing osteomyelitis.

Typhoid and enteric fever, Salmonella Typhi and Paratyphi, is a systemic febrile illness following ingestion. The organism disseminates via the bloodstream and reticuloendothelial system, Peyer's patches, liver, spleen, bone marrow, rather than causing a primarily diarrhoeal illness initially.

It presents classically with:

• a stepwise rising fever
• relative bradycardia, a pulse-temperature dissociation
• abdominal symptoms, which may include constipation rather than diarrhoea, particularly early
• hepatosplenomegaly
• in some patients, a faint truncal rose spot rash

Complications include intestinal haemorrhage and perforation, from Peyer's patch ulceration, in untreated or advanced disease. A chronic gallbladder carrier state, classically illustrated by Typhoid Mary, can persist long after clinical recovery, with the organism residing in the gallbladder, particularly in the presence of gallstones, and being intermittently shed in stool.

Diagnosis. Stool culture for nontyphoidal disease. Blood culture is the more sensitive test for typhoidal disease, particularly early in the illness, since the organism is bacteraemic before it is reliably recovered from stool. Bone marrow culture offers the highest sensitivity for typhoid but is reserved for specific diagnostic difficulty given its invasiveness.

Prevention. Typhoid vaccines, both injectable polysaccharide and oral live-attenuated formulations, are available for travellers to endemic areas and for outbreak control. Safe food handling and poultry and egg hygiene are the principal measures against nontyphoidal disease.

Morphology. Gram negative bacilli, nonmotile, a key distinguishing feature from the otherwise biochemically similar Salmonella and from E. coli.

Key biochemical features. Lactose nonfermenting, urease negative, H2S negative, the key distinguishing feature from Salmonella on selective media.

Reservoir. Human gastrointestinal tract exclusively. No significant animal reservoir, and the organism does not survive well outside a human host, a point directly connected to its very low infectious dose.

Transmission. Faecal-oral, notable for an extremely low infectious dose, as few as 10 to 100 organisms, reflecting the organism's marked acid resistance allowing it to survive passage through the stomach. This low infectious dose makes person-to-person transmission, particularly in settings of close contact and suboptimal hygiene, such as childcare centres, highly efficient, distinct from most other enteric pathogens, which typically require a much larger inoculum via heavily contaminated food or water.

Virulence factors. Invasion of colonic epithelial cells via a type III secretion system, followed by intracellular replication and direct cell-to-cell spread via actin-based motility. Shiga toxin, produced classically by Shigella dysenteriae type 1, the most severe Shigella species, is mechanistically identical in action to the Shiga toxin of enterohaemorrhagic E. coli, and capable of causing haemolytic uraemic syndrome via the same mechanism.

Disease patterns. Shigellosis, bacillary dysentery, is an inflammatory, invasive colitis producing frequent, small-volume, bloody, mucoid stools, dysentery, abdominal cramping, tenesmus, and fever. It is distinguishable clinically from the typically larger-volume watery diarrhoea of noninvasive pathogens such as ETEC or cholera.

It can rarely be complicated by haemolytic uraemic syndrome, specifically with S. dysenteriae type 1, and by reactive arthritis, part of the broader postenteric reactive arthritis association shared with Salmonella, Campylobacter, and Yersinia.

Diagnosis. Stool culture on selective media. Stool microscopy classically shows abundant faecal leukocytes, reflecting the invasive, inflammatory nature of the infection, a useful bedside distinguishing clue from noninvasive diarrhoeal illness.

Resistance patterns. Antibiotic treatment, when indicated, generally for moderate-to-severe disease or in specific high-risk patients, shortens illness duration and reduces transmission, in contrast to the general avoidance of antibiotics in most nontyphoidal Salmonella gastroenteritis. Increasing antimicrobial resistance is a recognised and monitored global concern for this organism.

Morphology. Gram negative coccobacilli and short bacilli.

Key biochemical features. Lactose nonfermenting or very slow fermenting, urease positive, characteristically motile at 25 degrees Celsius but nonmotile at 37 degrees Celsius, an unusual temperature-dependent motility pattern.

Reservoir. Broad animal reservoir, particularly pigs, with pork and pork products, including undercooked pork and contaminated chitterlings, being a classically associated transmission source. Also found in contaminated water and unpasteurised dairy.

Transmission. Faecal-oral, via contaminated food, particularly pork products, or water. Notably, the organism's ability to grow, albeit slowly, at refrigeration temperatures is relevant to transmission via contaminated, refrigerated food products, including, rarely, contaminated blood products stored under refrigeration, a recognised, though uncommon, cause of transfusion-transmitted infection.

Virulence factors. Invasion of intestinal epithelium, particularly overlying the terminal ileal Peyer's patches, with a particular tropism for lymphoid tissue that produces the organism's most clinically distinctive presentation below. Type III secretion system-mediated effector protein injection is similar in general principle to Salmonella and Shigella.

Disease patterns. Enterocolitis, typically in young children, presents with diarrhoea, occasionally bloody, fever, and abdominal pain.

The organism's particular tropism for the terminal ileum and mesenteric lymph nodes classically produces mesenteric lymphadenitis, a presentation that can closely mimic acute appendicitis clinically, with fever and right lower quadrant pain. This diagnostic mimic is sometimes called pseudoappendicitis, and is particularly relevant in older children and young adults.

Postenteric complications include reactive arthritis and, less commonly, erythema nodosum.

Diagnosis. Stool culture, though the organism grows more slowly than most other stool pathogens and may require cold enrichment or specific selective media, CIN agar, for optimal recovery. Clinical suspicion of pseudoappendicitis in a patient found to have a normal appendix at surgery, or in whom imaging suggests mesenteric lymphadenitis rather than appendicitis, should prompt specific consideration of this organism.