Bacterial filamentation of Yersinia pestis by beta-lactam antibiotics in experimentally infected mice.
To identify alternatives to streptomycin for treating pneumonic plague, we evaluated beta-lactam antibiotics to treat experimental pneumonic plague in mice. Mice were exposed to a lethal inhaled dose of Yersinia pestis and treated with beta-lactam antibiotics administered every 6 hours, starting 42 hours postexposure. The mice died or were euthanized in extremis 3 days postexposure. We observed marked bacterial filamentation of Y pestis in the tissues of mice treated with ceftazidime (10/10 mice), aztreonam (9/10 mice), or ampicillin (1/10 mice), but not in the tissues of mice treated with cefotetan, cefazolin, ceftriaxone, or saline. There was no evidence of septation of the filamentous bacteria by light or electron microscopy. The filamentous bacteria were confirmed as Y pestis based on their reactivity with rabbit anti-Y pestis F1 serum. Marked bacterial filamentation of Y pestis can be produced in vivo by certain beta-lactam antibiotics. This antibiotic-induced morphologic change is important because filamentous bacteria in clinical samples could possibly be confused with filamentous actinomycotic organisms.