Surveillance of human salmonellosis in Bulgaria, 1999-2004: trends, shifts and resistance to antimicrobial agents

Abstract

This article analyses the distribution of resistant salmonella and resistance mechanisms among the most frequently encountered serotypes in Bulgaria. Culture, biochemical tests and serotyping were used for identification. Screening for resistance to 14 antimicrobial agents with the standard Bauer-Kirbi disk-diffusion method. The double disk synergy method was used to determine production of extended-spectrum a-lactamases (ESBL). Transfer of genes coding for ESBLs with experimental conjugation. Specific primers were used for PCR detection of bla-CTX-M, bla-SHV and bla-TEM. 245 resistant salmonella strains were determined in our study; the majority originated from sporadic cases of human illness or asymptomatic infection and the remaining 23 were isolated from outbreaks. 79 producers of ESBL were detected: 5 S. Enteritidis, 1 S. Typhimurium, 9 S. Isangi and 62 S. Corvallis with types of enzymes: CTX-M3, TEM and SHV. Gene coding for extended-spectrum a- lactamases were successfully transferred into a recipient Escherichia coli C1A strain simultaneously with genes coding for resistance to aminoglycosides and sulphonamides (for bla-CTX-M3) and gene coding for resistance to aminoglycosides and chloramphenicol (for bla-SHVand bla-TEM). PCR amplification revealed bla-CTX-M3 genes in S. Enteritidis, and bla-SHV and bla-TEM in S. Corvallis. Salmonellae have revealed increasing resistance to all clinically important groups of antimicrobial agents. Bulgaria is the first country in the world where ESBL in serotype Corvallis has been reported. A wide diversity of resistance genes is found among the leading serotypes of salmonella causing human disease in Bulgaria.