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Tag words: bacteria, enteric bacteria, microbiology, microbe, Salmonella, Salmonella Enterica, Salmonella Typhi, S. Typhimurium, S. Enterica, typhoid fever, enteric fever, salmonellosis, food poisoning, gastroenteritis.


Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Enterobacteriales
Family: Enterobacteriaceae
Genus: Salmonella
Species: e.g. S. enterica

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

Bacillus cereus bacteria.Print this Page

Salmonella and Salmonellosis (page 5)

(This chapter has 5 pages)

© Kenneth Todar, PhD

Antibiotic Susceptibility

During the last decade, antibiotic resistance and multiresistance of Salmonella spp. have increased a great deal. The cause appears to be the increased and indiscriminate use of antibiotics in the treatment of humans and animals and the addition of growth-promoting antibiotics to the food of breeding animals. Plasmid-borne antibiotic resistance is very frequent among Salmonella strains involved in pediatric epidemics (e.g., Typhimurium, Panama, Wien, Infantis). Resistance to ampicillin, streptomycin, kanamycin, chloramphenicol, tetracycline, and sulfonamides is commonly observed. Colistin resistance has not yet been observed.

Until 1972, Typhi strains had remained susceptible to antibiotics, including chloramphenicol (the antibiotic most commonly used against typhoid) but in 1972, a widespread epidemic in Mexico was caused by a chloramphenicol-resistant strain of S. Typhi. Other chloramphenicol-resistant strains have since been isolated in India, Thailand, and Vietnam. Possible importation or appearance of chloramphenicol-resistance strains in the United States is a real threat. Salmonella strains should be systematically checked for antibiotic resistance to aid in the choice of an efficient drug when needed and to detect any change in antibiotic susceptibility of strains (either from animal or human source). Indiscriminate distribution and use of antibiotics should be discouraged.

Vaccination Against Typhoid Fever

Three types of typhoid vaccines are currently available for use in the United States: (1) an oral live-attenuated vaccine; (2) a parenteral heat-phenol-inactivated vaccine;  (3) a newly licensed capsular polysaccharide vaccine for parenteral use.  A fourth vaccine, an acetone-inactivated parenteral vaccine, is currently available only to the armed forces.

1. Live oral vaccines. Although oral killed vaccines are without efficacy, vaccines using living avirulent bacteria have shown promise. A galactose-epimeraseless mutant of Typhi has given very good results in a field trials. Mutants of Typhimurium that have given a good protection in animals include mutants lacking adenylate-cyclase and AMP receptor protein, and  mutants auxotrophic for p-aminobenzoate and adenine.Typhi with the same mutations does not cause adverse reactions and is immunogenic in human.

The Live Oral Typhoid Vaccine should not be given to children younger than 6 years of age. It is given in four doses, 2 days apart, as needed for protection.  The last dose should be given at least 1 week before travel to allow the vaccine time to work. A booster dose is needed every 5 years for people who remain at risk.

2. The parenteral heat-phenol-inactivated vaccine has been widely used for many years. In field trials involving a primary series of two doses of heat-phenol- inactivated typhoid vaccine, efficacy over the 2- to 3-year follow-up periods ranged from 51% to 77% . Efficacy for the acetone- inactivated parenteral vaccine, available only to the armed forces, ranges from 75% to 94%.

Since the inactivated vaccines contain the  O antigen (endotoxin), local and general reactions occur. Vi antigen extracted following the methodology used for the meningococcal vaccine seems to avoid reactions to endotoxin.

The inactivated Typhoid Vaccine should not be given to children younger than 2 years of age. One dose provides protection.  It should be given  at least 2 weeks before travel to allow the vaccine time to work. A booster dose is needed every 2 years for people  who remain at risk.

3. The newly licensed parenteral vaccine [Vi capsular polysaccharide (ViCPS)] is composed of purified Vi ("virulence") antigen, the capsular polysaccharide elaborated by S.Typhi isolated from blood cultures. In recent studies, one 25-ug injection of purified ViCPS produced seroconversion (i.e., at least a fourfold rise in antibody titers) in 93% of healthy U.S. adults.  Two field trials in disease-endemic areas have demonstrated the efficacy of ViCPS in preventing typhoid fever. In one trial in Nepal, in which vaccine recipients were observed for 20 months, one dose of ViCPS among persons 5-44 years of age resulted in 74%  fewer cases of typhoid fever. ViCPS has not been tested among children less than 1 year of age.

NOTE: No typhoid vaccine is 100% effective and is not a substitute for being careful about what you eat or drink.

Routine typhoid vaccination is not recommended in the United States, but typhoid vaccine is recommended for travellers to parts of the world where typhoid is common, people in close contact with a typhoid carriers, and laboratory workers who work with Salmonella Typhi bacteria.

Figure 6. Salmonella that has been cultured in a tetrathionate-enrichment broth, and stained using the direct fluorescent-antibody (FA) technique. Tetrathionate-enrichment broth contains bile salts, thereby inhibiting the growth of Gram-positive organisms, while the Gram-negative Salmonella sp., being an organism that possess the enzyme tetrathionate reductase, is able to break down tetrathionate, and grow uninhibited. (CDC)

For more information on salmonella infections please see
CDC  Salmonellosis
CDC Salmonella enteritidis
CDC  Salmonella Infection (salmonellosis) and Animals
CDC Typhoid General information
CDC Typhoid traveller's information
CDC  Typhoid vaccine: What You Need to Know
FDA/CFSAN Bad Bug Book - Salmonella spp
MedlinePlus Enteric fever
NOVA | The Most Dangerous Woman in America (Typhoid Mary)
Typhoid and paratyphoid fever (UK)
Typhoid Fever Utah Health Dept
Typhoid fever NY Communicable Disease Fact Sheet
WHO Salmonella
WHO Typhoid fever


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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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