We studied the mechanism of resistance to imipenem in three clinical isolates of Pseudomonas aeruginosa. Two of these isolates arose from imipenem-susceptible strains isolated during therapy with imipenem and were associated with treatment failure. One of these two strains had previously been broadly resistant to beta-lactams; the second acquired resistance to imipenem alone. One isolate of the third strain was resistant to imipenem but susceptible to other antipseudomonal beta-lactams. No isolate contained beta-lactamase activity capable of hydrolyzing imipenem at a detectable rate. Studies of the penicillin-binding proteins of all isolates revealed no differences in the number of proteins, molecular weight of, affinity for penicillin, or affinity for imipenem in any isolate. In each case the resistant isolate lacked one or more outer membrane proteins that were present in a susceptible isolate of the same strain. The observed alterations in outer membrane proteins may be associated with diminished permeability of the bacterial outer membrane to imipenem and may be the major factor responsible for resistance in these isolates.

With the advent of high-throughput whole-genome sequencing, it is now possible to sequence a bacterial genome in a matter of hours. However, although the presence or absence of a particular gene can be determined, we do not yet have the tools to extract information about the true virulence potential of an organism from sequence data alone. Here, we focus on the important human pathogen Staphylococcus aureus and present a framework for the construction of a broad systems biology-based tool that could be used to predict virulence phenotypes from S. aureus genomic sequences using existing technology.

MATERIALS AND METHODS Evaluation of patients. Between 1 January 1985 and 31 March 1985, 226 patients whose expectorated sputum in culture yielded potential pathogens were evaluated for clinical and radiologic evidence of upper or lower respiratory tract infection. An additional 23 patients were identified as

We investigated the mechanism of resistance to penicillin in two penicillin-resistant clinical isolates of viridans streptococci that caused life-threatening infections in two patients not receiving chronic penicillin therapy. The first was a strain of Streptococcus intermedius that was isolated from the cerebrospinal fluid of a patient with post-neurosurgical meningitis. The second was a strain of Streptococcus mitis recovered from the bloodstream of a leukemic patient with neutropenia. Both patients failed to respond to penicillin. The mechanism of resistance in these strains was associated with diminished affinity for penicillin of their penicillin-binding proteins, as compared with those of penicillin-susceptible control strains. We conclude that penicillin-resistant viridans streptococci may cause serious infections even in patients not receiving chronic penicillin therapy, that this resistance is clinically significant and may result in failure of penicillin therapy, and that the mechanism of resistance in these strains is associated with diminished affinity of the penicillin-binding proteins for penicillin.