Highly Susceptible Strains of Typhoid Bacilli Encountered in Jamaica

Abstract

Background: Unlike reports of multidrug-resistant Salmonella enterica serovar typhi (S. typhi) in countries around the world, strains encountered in Jamaica have been uniformly susceptible to all the anti-typhoid drugs and also to other antibiotics by disc method. We have been maintaining these isolates on Dorset-egg medium over the years. In this report, we examine the MICs of 4 front-line antibiotics against 41 unduplicated isolates(one from a patient) encountered in consecutive 17 years between 1984 and 2000 at the University Hospital in Kingston, Jamaica. Methods: The MICs were determined by E test(AB Biodisk, Solna, Sweden) using E. coli ATCC 25922 as control. Manufacturers’ instructions in regard to media, inoculum density and incubation parameters were followed strictly. Our observation of extremely low MICs (see results below) made us to do the tests repeatedly and read the results independently by each of us (NCB and OH) and repeat again if we differed in reading by more than one E test dilution. Results: The MICs( g/ml) of the four antibiotics were Chloramphenicol MIC range 2—4, MIC50 3 and MIC90 3; Ampicillin MIC range 0.125—1, MIC50 0.25 and MIC90 0.5; Trimethprim/Sulpha MIC range 0.023—0.064, MIC50 0.032 and MIC90 0.047; Ceftriaxone MIC range 0.023—0.047, MIC50 0.032 and MIC90 0.047. All isolates were susceptible. MICs were extremely low, fell in a narrow range and far below the standard susceptible(CLSI) Breakpoint MICs of the antibiotics. We have not seen any report of such a highly susceptible strains of typhoid bacilli from anywhere in the world. Conclusion: Considering the growing increase of multidrug-resistant typhoid in countries around the world and reports of isolation of strains with MICs of front-line antibiotics of more than 256 g/ml(Hirose K et al Antimicrob Ag Chemother 45:956—958,2001), the highly susceptible nature of strains encountered in Jamaica is noteworthy. These unique strains which we call ‘Jamaica strains’ have been persisting in this island country throughout the years.