Study of resistance mechanism on cefotaxime resistant Proteus mirabilis isolated from clinical specimens and its clinical background

: Over a 6-year period (1997 to 2002), 56 strains of Proteus mirabilis (12% of the total number of P. mirabilis isolates obtained) resistant to ampicillin, piperacillin, cefazolin and cefoperazone by routine antimicrobial testing method, were isolated in Saitama Medical School Hospital. Of the 56 strains resistant to 4 beta-lactams, 12 strains were studied and were found to produce extended-spectrum beta-lactamases, identified as CTX-M-10 group and Toho-1 group in 8 and 2 strains, respectively. Susceptibility testing showed that 12 strains were resistant to cefotaxime, and cepodoxime, and ceftriaxon but susceptible to ceftazidime. Moreover, all of the beta-lactamases were inhibited by clavulanic acid. Of the 12 strains, one strain showed resistance to cephamycins such as cefoxitin, cefmetazole and cefotetan. Four of the twelve patients had infections caused by ESBL producing P. mirabilis, and eight patients were colonized, as confirmed by clinical and laboratory findings. The infections were urinary tract infections (two episodes), pneumonia (one episode), and sepsis (one episode). These patients had a favorable response to antibiotic therapy including cephalosporin. From these findings, CTX-M-type beta-lactamase producing P. mirabilis strains were confirmed from clinical specimens in our hospital.