Extracellular protein toxins contribute to the pathogenesis of a wide variety of Staphylococcus aureus infections. The present study investigated the effects that cell-wall active antibiotics and protein-synthesis inhibitors have on transcription and translation of genes for Panton-Valentine leukocidin, alpha-hemolysin, and toxic-shock syndrome toxin 1, in both methicillin-sensitive and methicillin-resistant S. aureus. Subinhibitory concentrations of nafcillin induced and prolonged mRNA for Panton-Valentine leukocidin, alpha-toxin, and toxic-shock syndrome toxin 1 and increased toxin production. In contrast, clindamycin and linezolid markedly suppressed translation, but not transcription, of toxin genes. These results suggest (1) that protein-synthesis inhibition is an important consideration in the selection of antimicrobial agents to treat serious infections caused by toxin-producing gram-positive pathogens and (2) that, by inducing and enhancing toxin production, inadvertent use of beta-lactam antibiotics to treat methicillin-resistant S. aureus infections may contribute to worse outcomes.

Severe invasive group A streptococcal (GAS) infections emerged in the late 1980s, yet no single virulence factor has been common to all isolates from infected patients. A strong association was recently found between isolates of such cases (regardless of M type) and the production of NAD glycohydrolase (NADase). Of interest, all M-1 strains isolated after 1988 were positive for NADase, whereas virtually all M-1 GAS were previously negative for NADase. Genetic analysis demonstrated that GAS isolates were >96% identical in nga and >99% identical in their upstream regulatory sequences. Furthermore, because NADase-negative strains did not produce immunoreactive NADase, we concluded that additional regulatory element(s) control NADase production. NADase purified from GAS altered neutrophil-directed migration and chemiluminescence responses and had potent ADP-ribosyltransferase activity. In summary, the temporal relationship of NADase expression, alone or with other streptococcal virulence factors, may contribute to the pathogenesis of invasive GAS infections.

Background. Community-acquired methicillin-resistant Staphylococcus aureus strains have recently been associated with severe necrotizing infections. Greater than 75% of these strains carry the genes for Panton-Valentine leukocidin (PVL), suggesting that this toxin may mediate these severe infections. However, to date, studies have not provided evidence of toxin production.Methods. Twenty-nine community-acquired methicillin-resistant Staphylococcus aureus and 2 community-acquired methicillin-susceptible S. aureus strains were collected from patients with infections of varying severity. Strains were analyzed for the presence of lukF-PV and SCCmecA type. PVL production in lukF-PV gene-positive strains was measured by ELISA, and the amount produced was analyzed relative to severity of infection.Results. Only 2 of the 31 strains tested, 1 methicillin-resistant Staphylococcus aureus abscess isolate and 1 nasal carriage methicillin-susceptible S. aureus isolate, were lukF-PV negative. All methicillin-resistant Staphylococcus aureus strains were SCCmec type IV. PVL was produced by all strains harboring lukF-PV, although a marked strain-to-strain variation was observed. Twenty-six (90%) of 29 strains produced 50-350 ng/mL of PVL; the remaining strains produced PVL in excess of 500 ng/mL. The quantity of PVL produced in vitro did not correlate with severity of infection.Conclusions. Although PVL likely plays an important role in the pathogenesis of these infections, its mere presence is not solely responsible for the increased severity. Factors that up-regulate toxin synthesis in vivo could contribute to more-severe disease and worse outcomes in patients with community-acquired methicillin-resistant Staphylococcus aureus infection.

Temperate bacteriophage Mx8 of Myxococcus xanthus encapsidates terminally repetitious DNA, packaged as circular permutations of its 49-kbp genome. During both lytic and lysogenic development, Mx8 expresses a nonessential DNA methylase, Mox, which modifies adenine residues in occurrences of XhoI and PstI recognition sites, CTCGAG and CTGCAG, respectively, on both phage DNA and the host chromosome. The mox gene is necessary for methylase activity in vivo, because an amber mutation in the mox gene abolishes activity. The mox gene is the only phage gene required for methylase activity in vivo, because ectopic expression of mox as part of the M. xanthus mglBA operon results in partial methylation of the host chromosome. The predicted amino acid sequence of Mox is related most closely to that of the methylase involved in the cell cycle control of Caulobacter crescentus. We speculate that Mox acts to protect Mx8 phage DNA against restriction upon infection of a subset of natural M. xanthus hosts. One natural isolate of M. xanthus, the lysogenic source of related phage Mx81, produces a restriction endonuclease with the cleavage specificity of endonuclease BstBI.Myxococcus xanthus is a representative of the myxobacteria, unicellular prokaryotes that display complex social behaviors. During vegetative growth, M. xanthus cells glide in swarms. When starved for nutrients, the swarms undergo a developmental program involving both the morphogenesis of a fruiting structure and the differentiation of a subset of cells into heatresistant spores. Both gliding motility and development depend on intercellular communication mediated by extracellular signal molecules. The pathways of signal transduction used by M. xanthus are simple prokaryotic models for similar pathways that mediate gliding motility and development in higher eukaryotes (5, 9, 21, 32).The study of M. xanthus cell-cell interactions has been facilitated by a variety of prokaryotic genetic methods. Among the most powerful of these methods is transduction, genetic exchange mediated by bacteriophage. M. xanthus is the host for both lytic (2) and lysogenic (13) generalized transducing phages. Our work has focused on the study of one of these phages, temperate phage Mx8. Mx8 particles have icosahedral capsids and short tails and encapsulate a terminally repetitious genome derived from 49 kb of unique, linear sequence (reference 13 and unpublished results).During lysogenic development, Mx8 integrates into the host chromosome at a preferred attachment site, attB. The prophage state is stable throughout development (15). Thus, Mx8 has the potential for being engineered as a cloning vector for host genes involved in M. xanthus motility and development. In this report, we describe the first step in the construction of specialized transducing derivatives of phage Mx8. We identify a nonessential region of the Mx8 genome that may be deleted to compensate for small insertions without the loss of terminal repetition. This region encodes a DNA adenine methylase, Mox, that is both nec...

An 8.1-kb fragment of the temperate Myxococcus xanthusphage Mx8 genome, when cloned into a plasmid vector, permits site-specific integration of the plasmid and confers superinfection immunity. Sequence analysis of a 9.5-kb region of Mx8 DNA containing this fragment reveals 19 densely packed open reading frames, four of which have predicted products with known or suspected activities. The Mx8 imm gene, required for superinfection immunity, has a sequence similar to that of Arabidopsis thalianaG-box-binding factor 1. Mx8 makes a DNA adenine methylase, Mox, and integrase, Int, related to other methylases and integrases. Theint gene has two alternate translation initiation codons within the extensively overlapping uoi (upstream of int) gene. Comparison of the predicted product of the uoi gene with Salmonella phage P22 and Streptomycesplasmid Xis proteins shows that temperate phage excisionases may use variations of a helix-turn-helix motif to recognize specific DNA sequences.