Chloramphenicol resistance in the typhoid bacillus

Area of Science:

• Microbiology
• Infectious Diseases
• Antimicrobial Resistance

Background:

• Salmonella typhi (S. typhi) resistance to chloramphenicol was noted in individual strains since 1950.
• Epidemics caused by resistant S. typhi strains emerged in 1972, with an outbreak in Mexico.
• A specific S. typhi strain from Mexico exhibited resistance to multiple antibiotics.

Purpose of the Study:

• To characterize the S. typhi strain responsible for the 1972 Mexican epidemic.
• To investigate the genetic basis of multidrug resistance in this S. typhi strain.
• To assess the implications of this resistant strain for public health.

Main Methods:

• Isolation and characterization of S. typhi strains from infected patients.
• Phenotypic antibiotic susceptibility testing.
• Plasmid analysis to identify resistance factors.
• Bacterial conjugation experiments to transfer R factors.

Main Results:

• The epidemic S. typhi strain was a degraded Vi strain resistant to chloramphenicol, streptomycin, sulfonamides, and tetracyclines.
• Resistance was mediated by a transferable R factor.
• The R factor could be transferred to Escherichia coli and subsequently to drug-sensitive S. typhi.

Conclusions:

• The emergence of multidrug-resistant S. typhi strains poses a significant public health threat, as evidenced by the 1972 Mexican epidemic.
• The transferability of R factors underscores the rapid spread of antimicrobial resistance.
• Judicious use of antibiotics, particularly chloramphenicol, is crucial to mitigate the development and spread of resistant bacterial strains.