Multidrug-resistant Pseudomonas aeruginosa in Brooklyn, New York:: molecular epidemiology and in vitro activity of polymyxin B

Multidrug-resistant strains of Pseudomonas aeruginosa have become increasingly problematic in certain hospitals. For a 3-month period in 2001, all unique patient isolates were collected from 15 hospitals in Brooklyn, New York, USA. Of 691 isolates, only 70% were susceptible to imipenem and 56% to ciprofloxacin. These susceptibility rates were lower than those found in a prior surveillance study in 1999 ( 76% and 71% susceptible to imipenem and ciprofloxacin, respectively; p< 0.001). The rate of imipenem resistance was associated with fluoroquinolone usage at each hospital ( p= 0.04). All isolates were susceptible to polymyxin B and 95% to amikacin. Among 195 imipenem- resistant isolates, 47 unique ribotypes were found. However, four ribotypes accounted for > 50% of isolates and were shared by most hospitals. Time-kill studies with 13 unique multiresistant strains revealed that polymyxin B was bactericidal against all strains at 4 mg/l, but only against 3 of 13 (23%) strains at 2 mg/l. Using 2 mg/l, significant bacterial regrowthwas evident for 5 of 13 (38%) strains. The addition of azithromycin to polymyxin B ( 2 mg/l) produced a mean decrease of 1 log cfu/ml greater than polymyxin alone and allowed bacterial regrowth in only 2 of 13 (15%) strains. Multiresistant P. aeruginosa is highly endemic to this city, with a few strains having spread among most hospitals. Polymyxin B remains active against all isolates and produces concentration-dependent killing in vitro. Azithromycin appears to enhance the in vitro activity of polymyxin B. The clinical utility of this combination remains to be established.