Clinical investigation of isolated bacteria from urinary tracts of hospitalized patients and their susceptibilities to antibiotics

Complicated urinary tract infections (UTIs) are often difficult to treat, partly because of the existence of isolated antibiotic-resistant strains. Even though the definition of UTI is determined by the quantity of cultured bacteria, it has been unclear if the quantity of cultured UTI bacteria affects their susceptibility to antimicrobial agents. Also, the gram stain is generally performed to assume the causative bacteria and their quantity before culture results can be obtained; therefore, we could start to use effective antibiotics if the relationship between bacterial quantity and resistance to antimicrobial agents were clear. We studied patients diagnosed as having complicated UTIs at the Urological Department in Kobe University Hospital between June 2002 and March 2003 and analyzed the relationships between the quantity of cultured bacteria and their antimicrobial MICs for antibiotics. The most commonly isolated bacteria were Serratia marcescens, Pseudomonas aeruginosa, Enterococcus faecalis, Escherichia coli, Staphylococcus epidermidis, Stenotrophomonas maltophilia, and methicillin-resistant Staphylococcus aureus. There was no significant correlation between the quantity of cultured bacteria and antimicrobial MICs in all the bacteria and antibiotics that we tested, suggesting that resistant phenotype, but not inoculum of the organism, did determine resistance to antibiotics. In conclusion, our investigation suggested the total number of isolated bacteria in urine culture did not determine the MICs and that inoculum of the bacteria might be important for this determination.