aureus, Enterococcus spp., and Candida spp. (18). The largest percentage increase (75.4%) in bloodstream bacterial infections occurred with CNS in large teaching hospitals. Bloodstream infections with Candida spp. increased in frequency by 487% in large teaching hospitals whereas those with S. aureus increased 176% and those with Enterococcus spp. increased 120%. Aerobic Gram-negative bacilli, which had achieved prominence in the previous two decades, showed no change in frequency in large teaching hospitals.

The spectrum of microbial pathogens involved overall in nosocomial infections differs somewhat from that causing only bacteremia/fungemia (19). Overall, in NNIS hospitals from 1986 to 1989, the five leading pathogens were E. coli (16%), Enterococcus spp. (12%), Pseudomonas aeruginosa (11%), S. aureus (10%), and CNS (9%). The leading role of E. coli reflects the frequency of urinary tract infections in hospitalized patients. Each of the other major categories of nosocomial infections has its own distinct leading pathogen: wound infection, S. aureus; pneumonia, P. aeruginosa and S. aureus; bloodstream infection, CNS. Over the past decade there has been a decline in the frequency of E. coli infections and increases in infections due to CNS (from 4% of total infections to 9%) and Candida albicans (from 2 to 5%). In the past decade the trend has been away from antimicrobial-susceptible species to more resistant pathogens. Even among susceptible species, there has been a shift from readily susceptible to antibiotic-resistant strains.

Overall data for a large number of hospitals are very helpful in identifying major trends. However, they may tend to obscure outbreaks that occur in individual hospitals and important shifts that occur in antimicrobial susceptibilities of pathogens that present problems in individual intensive-care units. For example, among 392 nosocomial infections occurring among 920 neutropenic cancer patients in a cancer unit, Candida spp. (18%) was the most common pathogen followed closely in frequency by CNS (17%) (10). In addition to common microorganisms seen in the multihospital NNIS survey, less frequently observed pathogens such as Clostridium spp., Corynebacterium spp., Aspergillus spp., and herpes viruses may account for 20% of the total pathogen group.

PROBLEMS OF ANTIMICROBIAL RESISTANCE IN SELECTED NOSOCOMIAL PATHOGENS

S. aureus

Methicillin-resistant S. aureus (MRSA) was first described in 1961 in England and over the next decade became an important nosocomial



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