Home » Microbiology » Salmonella


Salmonella is a genus of gram negative rods, which are motile, non-spore forming and non-encapsulated facultative anaerobes. These bacteria cause typhoid fever, enterocolitis and septicemia.


Route of Entry
•    Ingestion of food and water contaminated by human and animal wastes
•    Salmonella Typhi is transmitted only by humans.
•    Most frequent animal source is poultry and eggs.
•    Undercooked meat is also a source

Dose of bacteria required is 100 million

Salmonella has all 3 antigens:
•     O or  somatic
•     H or flagella
•     K or capsular

Somatic antigen contains lipopolysaccharide toxin.

Virulence Factors
Important virulence factors include:
•    Pili
•    Capsule
•    Endotoxin- Lipopolysaccharide
•    Enterotoxin
•    Protein invasion

Invasion allows non-phagocytic cells to take up bacteria, where bacteria are able to live intracellularly.

Predisposing Factors
•    Patients suffering from chronic  diseases
•    Low hygiene
•    Pets e.g dogs
•    Undercooked  meat
•    Unpasteurized  milk or dairy products

Lab- Diagnosis

•    Blood            during first 10 days of infection
•    Faces             during 2nd -3rd week of infection
•    Urine             during 2nd  week of infection

Color-enhanced scanning electron micrograph showing Salmonella Typhimurium (red) invading cultured human cells
Color-enhanced scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells

Salient features seen on microscopy are:
•    Gram negative rods
•    Motile
•    Non-spore forming
•    Non- capsulate expect
•    Facultative  anaerobe
•    Pili are present


Bacteria may be grown on:

1.    Blood Agar 
Grey-white colonies are formed, which are non- hemolytic.

2.    XLD         
a.    H2S producers form pink, red colonies with black centers
b.    Non- H2S producers form pink, red colonies without black centers

3.    MacConkey’s agar
Colorless colonies are formed.

Biochemical Tests

Triple sugar iron (TSI) test
On triple sugar iron test, alkaline slant and acidic butt is produced. H2S gas may also be produced.

•    Urease                      Negative
•    Indole                       Negative
•    Lactase                     Negative
•    B. Galactosidase     Negative
•    Oxidase                    Negative
•    Urine                        Nitrate test positive i.e. converts nitrates to nitrites
•    Ferment                   Glucose

Serological Tests

•    Widal Test- detecting a rise in antibody titer in patient’s serum
•    IGM  antibody Immunoassays
•    PCR


Enterocolitis is characterized by an inversion of the epithelial and sub epithelial tissue of the small and large intestine, strains penetrate both through and between the mucosal cells in to the lamina propria, causing inflammation and diarrhea.
Asymptomatic carriage in gallbladder takes place as well.
Carriers excrete 1-1000 million Salmonella typhi /gm in faces and urine.

Infection beings in the small intensive but without gastrointestinal symptoms of peyer’s patches, then spreads to the phagocytes of the liver, gallbladder and spleen.

In bacteremia, fever and other symptoms are caused by endotoxin.

Organisms if present within phagosomes in phagocytes of the gallbladder, leads to carrier state.

Septicemia usually occurs in patients with underlying chronic diseases e.g sickle cell anemia or cancer or enterocolitis.

Clinical Symptoms

Diarrhea, vomiting, fever and abdominal pain within 12-36 hours after ingestion of infected food.

Osteomyelitis, pneumonia and meningitis are the main symptoms.

Enteric Fevers
Main symptoms include typhoid, persistent high fever, low pulse rate, severe headache, toxemia, enlargement of the spleen, nausea, mental confusion and rose spots.


•    Enterocolitis is self limited disease, thus no treatment is required.
•    Antimicrobials are needed for neonates and chronically ill patients
•    In typhoid fever ciprofloxacin is given

Check Also

Macroconidia of Epidermophyton floccosum and filamentous hyphae

Cutaneous Mycoses

Cutaneous mycoses are the diseases caused by fungi and involving the skin, hair and nails. …

Leave a Reply

Your email address will not be published. Required fields are marked *