Enterobacterales

Salmonella spp.

Gram-negative bacilli

Identification & Growth

Identification

Oxidase-negative, lactose-negative, H₂S-positive, lysine-positive, indole-negative, urease-negative
Serological confirmation with O and H antisera

Growth Conditions

SS and Hektoen agar: colonies with a black centre; XLD agar: red colonies with a black centre
Enrichment in selenite or tetrathionate broth; 35–37 °C, 24 h

Clinical Significance

Outpatient gastroenteritis, typhoid fever, bacteremia in immunosuppressed patients

Intrinsic Resistance

Penicillin G, macrolides, clindamycin, glycopeptides, linezolid, daptomycin

Bench Alerts

Aminoglycosides and first-/second-generation cephalosporins may appear susceptible in vitro but fail clinically — do not report
Screen for reduced fluoroquinolone susceptibility with nalidixic acid/pefloxacin
Notifiable disease in outbreak settings

Clinical Notes

Clinical presentations

Non-typhoidal Salmonella causes self-limited gastroenteritis, typically with diarrhea, fever, and abdominal cramps.
Salmonella Typhi and Paratyphi cause systemic enteric fever with sustained bacteremia and possible intestinal perforation.
Extraintestinal bacteremia with metastatic foci (osteomyelitis, mycotic aneurysm) occurs more in immunocompromised patients and those with hemoglobinopathies.
Chronic asymptomatic carrier state may persist in the gallbladder after S. Typhi infection.

Specimens and collection

Stool culture is the standard specimen for gastroenteritis, preferably collected during the acute diarrheal phase.
Blood cultures are essential for diagnosing enteric fever, with higher sensitivity in the first week of illness.
Bone marrow culture has higher sensitivity than blood culture for typhoid fever, especially after prior antibiotic use.
Specimens should be transported in Cary-Blair medium when processing is delayed.

Epidemiology and at-risk populations

Transmission is predominantly fecal-oral, associated with contaminated food, especially eggs, poultry, and unpasteurized dairy.
Enteric fever is endemic in regions with poor sanitation, mainly South Asia and sub-Saharan Africa.
Patients with sickle cell disease have increased risk of Salmonella osteomyelitis.
Elderly patients, infants, and those with HIV have higher risk of invasive disease and bacteremia.

Resistance and therapeutic implications

Resistance to fluoroquinolones and third-generation cephalosporins is increasing, particularly in strains associated with international travel.
Extensively drug-resistant (XDR) Salmonella Typhi strains have been described in Pakistani outbreaks.
Uncomplicated gastroenteritis generally does not require antimicrobial treatment, reserved for severe cases or those at risk of bacteremia.

Bench and reporting notes

Selective agar such as XLD or Salmonella-Shigella (SS) aids differentiation from other fecal Enterobacterales.
Serotyping (Kauffmann-White scheme) and serogroup identification should be referred to a reference laboratory for epidemiologic surveillance.
Isolates from blood or sterile sites by non-typhoidal Salmonella should be reported as a potential public health event.

Sources

Educational decision-support content. It does not replace laboratory validation, current guidelines or review by the responsible professional.

FAQ: Frequently Asked Questions

How to identify Salmonella spp. in the lab?

Salmonella spp. is identified through oxidase-negative, lactose-negative, h₂s-positive, lysine-positive, indole-negative, urease-negative, serological confirmation with o and h antisera.

What are the intrinsic resistances of Salmonella spp.?

This organism is naturally resistant to penicillin g, macrolides, clindamycin, glycopeptides, linezolid, daptomycin. These drugs should not be reported as susceptible.

Where is Salmonella spp. commonly found?

It is typically associated with outpatient gastroenteritis, typhoid fever, bacteremia in immunosuppressed patients.

Antibiogram InterpreterCheck BrCAST 2026 breakpoints Rugai InterpreterBiochemical identification tools