Microbial resistance surveillance techniques. Blood culture versus multiple body site monitoring

The continuing problem of nosocomial bacterial infections has resulted in the development of new techniques to monitor their prevalence and detect the emergence of drug resistance. "Focused microbial surveillance," in which bacterial incidence and antimicrobial susceptibilities are assessed by specific hospital units, provides unique information in the evaluation of emerging outbreaks of resistant Gram-negative pathogens. Within a given unit, however, pooled microbiologic data may be misleading, since they reflect only the average of multiple susceptibilities and may represent either colonization or infection. Formerly, at the University of Iowa Hospitals and Clinics, bacterial surveillance within a particular unit had typically been performed using blood-sample analysis, which was viewed as an effective index of the activity of most classes of antimicrobial agents. However, specific body site surveillance (wounds, sputum, urine, and so forth) may offer advantages over blood-culture analysis with some forms of resistance (or types of bacteria), such as stably derepressed type-I beta-lactamase production, which seems to develop rather quickly. Site-specific surveillance may enable earlier detection of rapidly emerging resistant strains and the identification of virulent new serotypes or specific causes. Our data demonstrated that there is a greater probability of detecting resistant bacterial strains of Enterobacter colonizing and subsequently producing infections in the respiratory tract, urinary tract, or wound site. There was a time delay of almost 3 years between first observing resistant strains in sputum, urine, or pus and encountering them in an alarming incidence among blood cultures. Other studies confirm this particular phenomenon. In a survey of penicillin-resistant enterococci, the majority of clinical isolates were obtained from sites other than blood: seven from urine; five from wounds; two each from rectal swabs, ascitic fluid, and blood; and one each from a peritoneal catheter, Bartholin's cyst abscess, and pancreatic abscess. Effective techniques for selective surveillance should be both sensitive and cost effective. Current evidence suggests that site-specific monitoring (wounds, sputum, pleural effusions, urine, and feces) offers the advantages of more rapid identification of resistance trends prior to their appearance in the bloodstream cultures.