Oral quinolones in hospitalized patients: an evaluation of a computerized decision support intervention

OBJECTIVE

To determine whether a computerized decision support system could increase the proportion of oral quinolone antibiotic orders placed for hospitalized patients.

DESIGN

Prospective, interrupted time-series analysis.

SETTING

University hospital in the south-eastern United States.

SUBJECTS

Inpatient quinolone orders placed from 1 February 2001 to 31 January 2003.

INTERVENTION

A web-based intervention was deployed as part of an existing order entry system at a university hospital on 5 February 2002. Based on an automated query of active medication and diet orders, some users ordering intravenous quinolones were presented with a suggestion to consider choosing an oral formulation.

MAIN OUTCOME MEASURE

The proportion of inpatient quinolone orders placed for oral formulations before and after deployment of the intervention.

RESULTS

There were a total of 15 194 quinolone orders during the study period, of which 8962 (59%) were for oral forms. Orders for oral quinolones increased from 4202 (56%) before the intervention to 4760 (62%) after, without a change in total orders. In the time-series analysis, there was an overall 5.6% increase (95% CI 2.8-8.4%; P < 0.001) in weekly oral quinolone orders due to the intervention, with the greatest effect on nonintensive care medical units.

CONCLUSIONS

A web-based intervention was able to increase oral quinolone orders in hospitalized patients. This is one of the first studies to demonstrate a significant effect of a computerized intervention on dosing route within an antibiotic class. This model could be applied to other antibiotics or other drug classes with good oral bioavailability.

Iron-cofactored superoxide dismutase inhibits host responses to Mycobacterium tuberculosis

Superoxide dismutase (SOD) is a ubiquitous metalloenzyme in aerobic organisms that catalyzes the conversion of superoxide anion to hydrogen peroxide. Mycobacterium tuberculosis is unusual in that it secretes large quantities of iron-cofactored SOD. To determine the role of SOD in pathogenesis, we constructed mutants of M. tuberculosis H37Rv with reduced SOD production. Compared with controls, SOD-diminished isolates were more susceptible to killing by hydrogen peroxide. The isolates were markedly attenuated, exhibiting nearly 100,000-fold fewer bacilli than virulent control strains in the lungs and spleens of C57BL/6 mice 4 wk after intravenous inoculation. In the lung, SOD-attenuated M. tuberculosis induced robust interstitial mononuclear cell infiltration within 24 h and many cells were apoptotic by TUNEL staining, whereas virulent H37Rv exhibited minimal early inflammatory response and only rare interstitial mononuclear cell apoptosis. During prolonged infections, C57BL/6 mice tolerated SOD-attenuated M. tuberculosis better than BCG, exhibiting 68% greater weight gain, quicker eradication of bacilli from the spleen, and less alveolar lung infiltration. These results establish the importance of SOD in the pathogenesis of tuberculosis. Its effect appears to be mediated in part by inhibiting innate host immune responses, including early mononuclear cell infiltration of infected tissues and apoptosis.

Enhanced production of recombinant Mycobacterium tuberculosis antigens in Escherichia coli by replacement of low-usage codons

A major obstacle to development of subunit vaccines and diagnostic reagents for tuberculosis is the inability to produce large quantities of these proteins. To test the hypothesis that poor expression of some mycobacterial genes in Escherichia coli is due, in part, to the presence of low-usage E. coli codons, we used site-directed mutagenesis to convert low-usage codons to high-usage codons for the same amino acid in the Mycobacterium tuberculosis genes for antigens 85A and 85B and superoxide dismutase. Replacement of five codons in the wild-type gene for antigen 85B increased recombinant protein production in E. coli 54-fold. The recombinant antigen elicited proliferation and gamma interferon production by lymphocytes from healthy tuberculin reactors and was recognized by monoclonal antibodies to native antigen 85, indicating that the recombinant antigen contained T-cell and B-cell epitopes. Northern blotting demonstrated only a 1.7- to 2.5-fold increase in antigen 85B mRNA, suggesting that the enhanced protein production was due primarily to enhanced efficiency of translation. Codon replacement in the genes encoding antigen 85A and superoxide dismutase yielded four- to sixfold increases in recombinant protein production, suggesting that this strategy may be generally applicable to overexpression of mycobacterial genes in E. coli.

Characterization of a chromosomal gene encoding type B beta-lactamase in phage group II isolates of Staphylococcus aureus

In contrast to most Staphylococcus aureus isolates in which the gene for staphylococcal beta-lactamase (blaZ) is plasmid borne, isolates typeable by group II bacteriophages frequently carry blaZ on the chromosome. Furthermore, the chromosomal gene encodes the type B variant of staphylococcal beta-lactamase for which the nucleotide and deduced amino acid sequences have not yet been reported. To better understand beta-lactamase production among phage group II staphylococci and the nature of the type B beta-lactamase, we determined the type and amount of enzyme produced by 24 phage group II isolates. Of these isolates, 1 did not produce beta-lactamase, 8 produced the type B enzyme, and 15 produced the type C enzyme. In all eight type B beta-lactamase-producing isolates, blaZ was located on the chromosome. This was in contrast to the type C beta-lactamase-producing isolates, in which blaZ was located on a 21-kb plasmid. The nucleotide sequence corresponding to the leader peptide and the N-terminal 85% of the mature exoenzyme form of type B S. aureus was determined. The deduced amino acid sequence revealed 3 residues in the leader peptide and 12 residues in the exoenzyme portion of the beta-lactamase that differ from the prototypic type A beta-lactamase sequence. These include the serine-to-asparagine change at residue 216 found in the kinetically similar type C enzyme, a threonine-to-lysine change at residue 128 close to the SDN loop (residues 130 to 132), and several substitutions not found in any of the other staphylococcal beta-lactamases. In summary, modern isolates of S. aureus typeable by group II phages produce type B or type C staphylococcal beta-lactamase. The type B gene resides on the chromosome and has a sequence that, when compared to the sequences of the other staphylococcal beta-lactamases, corresponds well with its kinetic properties.

Synergism between poly-(1-6)-beta-D-glucopyranosyl-(1-3)-beta-D-glucopyranose glucan and cefazolin in prophylaxis of staphylococcal wound infection in a guinea pig model

To determine whether the infection-preventing capability of the neutrophil-activating agent poly-(1-6)-beta-D-glucopyranosyl-(1-3)-beta-D-glucopyranose glucan (PGG-glucan) can be enhanced with antibiotic prophylaxis, we administered PGG-glucan and cefazolin, alone and in combination, to guinea pigs inoculated with isolates of staphylococci. Guinea pigs receiving both PGG-glucan and cefazolin had 50% infective doses that were 8- to 20-fold higher than those obtained with cefazolin alone and 100- to 200-fold higher than those obtained with PGG-glucan alone. PGG-glucan and cefazolin are synergistic in their ability to prevent staphylococcal wound infection.

Recombinant expression and characterization of the major beta-lactamase of Mycobacterium tuberculosis

New antibiotic regimens are needed for the treatment of multidrug-resistant tuberculosis. Mycobacterium tuberculosis has a thick peptidoglycan layer, and the penicillin-binding proteins involved in its biosynthesis are inhibited by clinically relevant concentrations of beta-lactam antibiotics. beta-Lactamase production appears to be the major mechanism by which M. tuberculosis expresses beta-lactam resistance. beta-Lactamases from the broth supernatant of 3- to 4-week-old cultures of M. tuberculosis H37Ra were partially purified by sequential gel filtration chromatography and chromatofocusing. Three peaks of beta-lactamase activity with pI values of 5.1, 4.9, and 4.5, respectively, and which accounted for 10, 78, and 12% of the total postchromatofocusing beta-lactamase activity, respectively, were identified. The beta-lactamases with pI values of 5.1 and 4.9 were kinetically indistinguishable and exhibited predominant penicillinase activity. In contrast, the beta-lactamase with a pI value of 4.5 showed relatively greater cephalosporinase activity. An open reading frame in cosmid Y49 of the DNA library of M. tuberculosis H37Rv with homology to known class A beta-lactamases was amplified from chromosomal DNA of M. tuberculosis H37Ra by PCR and was overexpressed in Escherichia coli. The recombinant enzyme was kinetically similar to the pI 5.1 and 4.9 enzymes purified directly from M. tuberculosis. It exhibited predominant penicillinase activity and was especially active against azlocillin. It was inhibited by clavulanic acid and m-aminophenylboronic acid but not by EDTA. We conclude that the major beta-lactamase of M. tuberculosis is a class A beta-lactamase with predominant penicillinase activity. A second, minor beta-lactamase with relatively greater cephalosporinase activity is also present.

Beta-lactamase production diminishes the prophylactic efficacy of ampicillin and cefazolin in a guinea pig model of Staphylococcus aureus wound infection

Clinical trials in surgery suggest that some failures of antibiotic prophylaxis are related to the in vivo degradation of beta-lactams by Staphylococcus aureus beta-lactamase. To explore this issue further, isogeneic isolates of S. aureus differing only in whether they contained the structural gene for type A staphylococcal beta-lactamase were constructed and compared for their ability to establish an abscess in a guinea pig model. With ampicillin prophylaxis, the ID50 was 870 cfu for the beta-lactamase-negative isolate VK7114 and 240 cfu for the beta-lactamase-producing isolate VK7115 (P < .001). Similarly, the ID50 was greater for the beta-lactamase-negative isolate when cefazolin prophylaxis was administered (599 vs. 128 cfu, VK7114 and VK7115; P < .001). In the setting of prophylaxis with beta-lactamase-susceptible antibiotics, beta-lactamase contributes to the pathogenesis of S. aureus wound infections.

Prophylactic anti-infective activity of poly-[1-6]-beta-D-glucopyranosyl-[1-3]-beta-D-glucopryanose glucan in a guinea pig model of staphylococcal wound infection

The judicious use of perioperative antibiotic prophylaxis reduces the infectious complications of surgery. However, increased bacterial resistance within hospitals may make antibiotic prophylaxis less effective in the future and alternative strategies are needed. New immunomodulatory agents might prevent wound infections by stimulation of the host immune system. To test this hypothesis, we administered poly-[1-6]-beta-D-glucopyranosyl- [1-3] -beta-D-glucopyranose glucan (PGG glucan), which enhances neutrophil microbicidal activity, intravenously to guinea pigs in doses ranging from 0.015 to 4 mg/kg of body weight on the day before, on the day of, and on the day after intermuscular inoculation with methicillin-resistant strains of Staphylococcus aureus and Staphylococcus epidermidis. Abscesses were identified at 72 h, and median infective doses (ID50) and statistical significance were determined by logistic regression. Guinea pigs receiving PGG glucan and inoculated with methicillin-resistant S. aureus and S. epidermidis exhibited ID50 of as much as 2.5- and 60-fold higher, respectively, than those of control guinea pigs not receiving PGG glucan. Maximal protection was observed with a dose of 1 mg of PGG glucan per kg, and efficacy was reduced at higher as well as at lower PGG glucan doses. Furthermore, a single dose of PGG glucan given 24 h following bacterial inoculation was found to be effective in preventing infection. We conclude that PGG glucan reduces the risk of staphylococcal abscess formation. Neutrophil-activating agents are a novel means of prophylaxis against surgical infection and may be less likely than antibiotics to be affected adversely by the increasing antibiotic resistance of nosocomial pathogens.

Association of borderline oxacillin-susceptible strains of Staphylococcus aureus with surgical wound infections

Staphylococcus aureus isolates which produce type A staphylococcal beta-lactamase have been associated with wound infections complicating the use of cefazolin prophylaxis in surgery. To further evaluate this finding, 215 wound isolates from 14 cities in the United States were characterized by antimicrobial susceptibility and beta-lactamase type and correlated with the preoperative prophylactic regimen. Borderline-susceptible S. aureus isolates of phage group 5 (BSSA-5), which produce large amounts of type A beta-lactamase and exhibit borderline susceptibility to oxacillin, comprised a greater percentage of the 120 wound isolates associated with cefazolin prophylaxis than they did of the 95 isolates associated with other prophylactic regimens (25% versus 12.6%, respectively; P < 0.05). In contrast, methicillin-resistant S. aureus isolates were distributed evenly between the two groups (8.3% versus 11.6%, respectively). In vitro assays demonstrated that cefazolin was hydrolyzed faster by BSSA-5 strains than by other beta-lactamase-producing, methicillin-susceptible strains (1.54 versus 0.50 microg/min/10(8) CFU, respectively; P < 0.0001). These data demonstrate that BSSA-5 strains are a distinct subpopulation of methicillin-susceptible S. aureus which frequently cause deep surgical wound infections. Cefazolin use in prophylaxis is a risk factor for BSSA-5 infection.

Expression of an antisense hla fragment in Staphylococcus aureus reduces alpha-toxin production in vitro and attenuates lethal activity in a murine model

Isogeneic bacterial strains that differ only in the production of a single microbial factor have been invaluable in studying the pathogenesis of bacterial infections. The targeted, intentional inactivation of a gene encoding a potential virulence determinant generally requires homologous recombination to replace the gene with an inactivated allele. To determine whether the insertion and expression of a fragment of a bacterial gene in an antisense orientation could be used as a rapid alternative to allelic inactivation for producing paired isogeneic isolates, we inverted a 600-bp fragment of the Staphylococcus aureus gene encoding alpha-toxin, hla, behind its native promoter on an Escherichia coli-S. aureus shuttle vector. A transformant of an S. aureus strain carrying the antisense hla fragment produced antisense hla RNA and made 16-fold less alpha-toxin than either its parent or an isogeneic transformant containing vector DNA without hla. Also, intraperitoneal injection of 1.5 x 10(9) CFU of the antisense hla-containing transformant was significantly less lethal in a murine model than that of the parent (1 of 10 versus 7 of 10 mice expired [P < 0.02]) or the transformant without hla (1 of 10 versus 7 of 7 mice expired [P < 0.001]). We conclude that the expression of a fragment of hla in an antisense orientation in S. aureus on a plasmid vector reduces alpha-toxin production and the lethal activity of the strain in a murine model. The antisense strategy for creating isogeneic strains of bacteria may facilitate molecular investigations into the pathogenesis of infection. It also may be useful in creating novel live-attenuated strains of bacteria for use as vaccine candidates.

Structure-function relationships among wild-type variants of Staphylococcus aureus beta-lactamase: importance of amino acids 128 and 216

beta-Lactamases inactivate penicillin and cephalosporin antibiotics by hydrolysis of the beta-lactam ring and are an important mechanism of resistance for many bacterial pathogens. Four wild-type variants of Staphylococcus aureus beta-lactamase, designated A, B, C, and D, have been identified. Although distinguishable kinetically, they differ in primary structure by only a few amino acids. Using the reported sequences of the A, C, and D enzymes along with crystallographic data about the structure of the type A enzyme to identify amino acid differences located close to the active site, we hypothesized that these differences might explain the kinetic heterogeneity of the wild-type beta-lactamases. To test this hypothesis, genes encoding the type A, C, and D beta-lactamases were modified by site-directed mutagenesis, yielding mutant enzymes with single amino acid substitutions. The substitution of asparagine for serine at residue 216 of type A beta-lactamase resulted in a kinetic profile indistinguishable from that of type C beta-lactamase, whereas the substitution of serine for asparagine at the same site in the type C enzyme produced a kinetic type A mutant. Similar bidirectional substitutions identified the threonine-to-alanine difference at residue 128 as being responsible for the kinetic differences between the type A and D enzymes. Neither residue 216 nor 128 has previously been shown to be kinetically important among serine-active-site beta-lactamases.

Nontropical pyomyositis in patients with AIDS

Nontropical pyomyositis in persons with acquired immunodeficiency syndrome (AIDS) is an unusual entity with only a few cases having been described in the United States. Staphylococcus aureus is the most common organism implicated. The infection usually presents in a subacute indolent fashion with minimal inflammation. Fever and leukocytosis may be absent, and blood cultures are frequently negative. The diagnosis usually can be established by a combination of clinical features, computed tomography or ultrasound, and prompt examination of material obtained by aspiration or debridement. This article describes two cases of S aureus pyomyositis in patients with AIDS and reviews the literature relevant to this infection.

Passive immunization with antiserum to a nontoxic alpha-toxin mutant from Staphylococcus aureus is protective in a murine model

A nonhemolytic, nonlethal variant of Staphylococcus aureus alpha-toxin constructed via oligonucleotide-directed mutagenesis and containing a single amino acid substitution (H-35 to L) was used to immunize a rabbit. The resulting antiserum was cross-reactive with wild-type alpha-toxin and neutralized its hemolytic activity in vitro. Passive immunization of mice with rabbit antiserum conferred protection against lethal challenge with wild-type alpha-toxin and against acute lethal challenge with a high-alpha-toxin -producing S. aureus strain. H35L alpha-toxin may be useful as a protective immunogen in S. aureus vaccine studies.

Growth of Staphylococcus aureus with nafcillin in vitro induces alpha-toxin production and increases the lethal activity of sterile broth filtrates in a murine model

The morbidity and mortality of Staphylococcus aureus infections remain high despite antibiotic therapy. To investigate further the observation that penicillins increase the hemolytic activity of staphylococcal cultures, 37 strains were grown in broth with and without subinhibitory nafcillin. Nafcillin stimulated hemolytic activity in nafcillin-susceptible and -resistant isolates. Sterile broth filtrates of nafcillin-associated cultures injected intraperitoneally in mice were more rapidly lethal than filtrates of the same strain grown without nafcillin. Lethality was neutralized by anti-alpha-toxin antisera. DNA-RNA hybridization revealed a nafcillin-associated increase in alpha-toxin mRNA during the postexponential growth phase after the activation of agr. Isolates grown in slightly inhibitory nafcillin concentrations had more alpha-toxin mRNA than did nafcillin-free cultures, whereas agr RNAIII levels were comparable. This suggests that nafcillin-induced alpha-toxin production is not entirely attributable to agr. A supplemental regulatory mechanism may be involved.

Production of A and C variants of staphylococcal beta-lactamase by methicillin-resistant strains of Staphylococcus aureus

Most methicillin-resistant Staphylococcus aureus (MRSA) strains produce beta-lactamase. To determine whether this enzyme(s) is identical to one or more of the four beta-lactamases produced by methicillin-susceptible strains, the beta-lactamases of 50 MRSA isolates were typed by using substrate profile analysis. Forty type A, no type B, ten type C, and no type D beta-lactamase-producing strains were identified. The beta-lactamase inhibitor sulbactam reduced the MICs of beta-lactamase-labile antibiotics, including ampicillin, penicillin G, and cefazolin, for type A and type C MRSA strains.

Comparative prophylactic efficacies of ciprofloxacin, ofloxacin, cefazolin, and vancomycin in experimental model of staphylococcal wound infection

Recent shifts in the species and antibiotic resistance patterns of bacteria causing nosocomial infections present new challenges for providing effective prophylaxis in surgery. Traditional regimens lack activity against methicillin-resistant staphylococci and many gram-negative species causing nosocomial infections. The new fluoroquinolones exhibit in vitro activity against many emerging surgical wound pathogens. To determine the potential of this class of antimicrobial agents for use in surgery, we compared the prophylactic efficacies of ciprofloxacin and ofloxacin with those of cefazolin and vancomycin in a guinea pig model of abscess formation. Four Staphylococcus aureus strains, one Staphylococcus epidermidis strain, and one Staphylococcus haemolyticus strain were evaluated. Vancomycin was the most effective prophylactic agent, exhibiting in vivo activity against all strains which was superior or equivalent to those of all other agents tested. Cefazolin was the least effective agent and surpassed the two quinolones in prophylactic efficacy against only one organism, a quinolone- and methicillin-resistant strain of S. aureus. The two quinolones provided excellent protection against infection with all but the quinolone-resistant isolate. The in vivo emergence of quinolone resistance among quinolone-susceptible isolates was not detected. The methicillin-resistant, quinolone-susceptible S. epidermidis and S. haemolyticus isolates were extremely susceptible to prophylaxis, exhibiting 50% infective doses above 4 x 10(6) CFU for seven of the eight antibiotic-strain combinations. We conclude that ciprofloxacin and ofloxacin may be effective antistaphylococcal agents in surgery. The role of these agents remains to be defined, and the definition should include consideration of an adverse effect upon antibiotic resistance patterns of organisms causing nosocomial infections.

Site-directed mutagenesis of the alpha-toxin gene of Staphylococcus aureus: role of histidines in toxin activity in vitro and in a murine model

Staphylococcus aureus alpha-toxin is a membrane-damaging exoprotein that oligomerizes to form transmembrane pores. Chemical modification of histidines with diethylpyrocarbonate has been shown to reduce the hemolytic activity of alpha-toxin, suggesting that one or more of the histidine residues is important for toxin function. To individually assess the functional importance of each of the four histidine residues (residues 35, 48, 144, and 259), we used oligonucleotide-directed mutagenesis of the cloned alpha-toxin gene to replace each histidine with leucine. The mutant toxins were expressed in S. aureus and evaluated for hemolytic activity in vitro and for lethality in an intraperitoneal murine model. Substitution of histidine 35 with leucine produced a mutant toxin (H35L) without hemolytic or lethal activity. Mutant toxins H48L, H144L, and H259L exhibited 7, 16, and 46%, respectively, of the hemolytic activity of wild-type toxin. Immunoblotting of purified H35L toxin incubated with liposomal membranes demonstrated intact membrane binding and hexamer formation that was clearly detectable but reduced compared with that of the wild-type toxin. This suggests that hexamer formation alone is not sufficient for the expression of alpha-toxin activity. The nature of the defect underlying the lack of activity of the H35L mutant toxin remains to be elucidated but may involve failure of the hexamer to span the lipid bilayer to form a transmembrane pore or a change in the internal surface and permeability characteristics of the pore.

Comparative prophylactic efficacy of cefazolin and vancomycin in a guinea pig model of Staphylococcus aureus wound infection

The increasing prevalence of methicillin-resistant Staphylococcus aureus as a wound pathogen in some institutions has prompted the use of vancomycin for surgical prophylaxis, although clinical data comparing vancomycin and cephalosporins are not available. A guinea pig model was used to compare the efficacy of vancomycin and cefazolin in preventing intermuscular abscess formation by 7 S. aureus strains. Both antibiotics were administered to achieve peak serum levels at the time of bacterial inoculation, and each remained > 1 micrograms/mL for a comparable duration. Vancomycin provided equivalent protection from infection by 1 methicillin-susceptible strain and significantly better protection against 4 methicillin-susceptible and both methicillin-resistant S. aureus strains. For most strains, the bacterial inoculum with a 50% probability of causing an abscess was 2 to 4 log10-fold higher with vancomycin than cefazolin prophylaxis. Prophylaxis with vancomycin is superior to cefazolin in preventing intermuscular infection by methicillin-susceptible and -resistant S. aureus.

Efficacy of prophylaxis with beta-lactams and beta-lactam-beta-lactamase inhibitor combinations against wound infection by methicillin-resistant and borderline-susceptible Staphylococcus aureus in a guinea pig model

Although some beta-lactams and beta-lactam-beta-lactamase inhibitor combinations exhibit activity against methicillin-resistant Staphylococcus aureus, there remains the concern that therapeutic failures may result from the selection of resistant subpopulations. The prophylactic use of these antibiotics in clean surgery, however, may prove adequate since wound infections arise from the inoculation of small numbers of bacteria. In this clinical setting, heterogeneity in the phenotypic expression of beta-lactam resistance may facilitate antibiotic efficacy. Similarly, beta-lactamase-mediated resistance in S. aureus is dependent on inoculum size, and it may be possible to prevent infection from small inocula with relatively labile beta-lactams. To test this hypothesis, antibiotics were administered to guinea pigs as prophylaxis against infection by two methicillin-resistant strains and one borderline-susceptible strain. Following prophylaxis with sulbactam or placebo, inoculation of only a dozen or fewer bacteria had a 50% probability of creating an abscess (50% infective dose [ID50]). The efficacy of ampicillin was similar to that of cefazolin, exhibiting moderate activity against the borderline-susceptible strain (ID50s, greater than 300 bacteria) and minimal activity against the methicillin-resistant strains (ID50s, fewer than 100 bacteria). Coadministration of sulbactam with ampicillin or cefazolin yielded better results than the beta-lactam alone for five of six strain-beta-lactam combinations, including an 80-fold increase in the efficacy of ampicillin-sulbactam compared with that of ampicillin for one methicillin-resistant strain (ID50s, 2,017 and 25 bacteria, respectively). Prophylaxis with beta-lactams, especially beta-lactam-beta-lactamase inhibitor combinations, reduces the risk of wound infection by beta-lactam-resistant S. aureus.

Low-inoculum model of surgical wound infection

A model of surgical wound infection that uses low inocula of bacteria and closely simulates clinical infection involved inoculating suspensions of Staphylococcus aureus and dextran microbeads into intermuscular sites on the dorsum of guinea pigs, harvesting lesions at 72-96 h, identifying as a positive end point lesions yielding staphylococci on subculture, and using logistic regression for data analysis. Prophylaxis was placebo, ampicillin, or cefazolin, and three representative strains of S. aureus were used. A highly significant correlation (P less than .001) was observed between inoculum sizes and infection rates. Without antimicrobial prophylaxis, ID50 for each strain was less than 10 organisms; with antimicrobials, ID50 was significantly higher. Differences in the virulence of strains and in the efficacy of the antimicrobial regimens also were observed. The model should prove useful for understanding mechanisms of virulence among pathogenic bacteria and for elucidating subtle but important differences in efficacy among antibiotics used in prophylaxis.

Characterization of four beta-lactamases produced by Staphylococcus aureus

Staphylococcus aureus produces four types of beta-lactamase (A, B, C, and D). To investigate the effect of specific beta-lactamase type upon staphylococcal resistance, each beta-lactamase was purified to homogeneity, and the Michaelis constants (Km values) and turnover numbers (kcat values) for various penicillin and cephalosporin substrates were determined. Whereas Km values of the four beta-lactamases were comparable for penicillin G, cephalothin, and cefamandole, the type A and D enzymes exhibited greater affinity than the type B and C beta-lactamases for nitrocefin, cefazolin, and cephapirin. Conversely, the type B and C beta-lactamases exhibited greater kcat values than the type A and D enzymes against most of the cephalosporin agents, excluding nitrocefin. In contrast to earlier reports suggesting that the type B beta-lactamase is relatively inefficient in hydrolyzing penicillin G, we found only minor differences in the specific activities and kcat values of the type A, B, and C beta-lactamases. The type D beta-lactamase was distinctly less active against penicillin G, however, exhibiting only 15 to 25% of the kcat values of the other beta-lactamases. More than a 2,000-fold difference between the relative efficiencies of hydrolysis (kcat/Km) of cefazolin and cefuroxime by the type A beta-lactamase exists. This greatly exceeds the 60-fold difference in the stability of penicillin G and cefazolin with the same enzyme. Whereas the isoelectric points of the type A, B, and C beta-lactamases were similar, the value for the type D beta-lactamase was distinguishably lower (10.1 for types A, B, and C and 9.7 for type D). We conclude that marked differences in the stability of commonly used beta-lactams to hydrolysis by the staphylococcal beta-lactamases are present. This heterogeneity and the clinical implication thereof need to be considered in the antibiotic management of staphylococcal infection.

Cefuroxime versus cefazolin as prophylaxis in vascular surgery

Although cefazolin prophylaxis has proven efficacy in vascular surgery, Staphylococcus aureus wound infections are still an important postoperative complication. In cardiac surgery, cefazolin's susceptibility to hydrolysis by staphylococcal beta-lactamase has been proposed to account for some prophylaxis failures. To determine whether the incidence of vascular wound infections can be reduced by administering a more beta-lactamase-stable cephalosporin, we undertook a prospective, randomized trial of cefuroxime versus cefazolin. Cefuroxime was administered as a 1.5 gm dose before operation and 750 mg every 3 hours during operation. Cefazolin was given as 1 gm before operation and 500 mg every 4 hours during operation. Both agents were continued every 6 hours after operation for 24 hours. Deep wound infections developed in seven of 272 (2.6%) cefuroxime and three of 287 (1.0%) cefazolin recipients (p = 0.2). Staphylococcus aureus wound infections occurred in five cefuroxime versus two cefazolin recipients. In vitro evaluation of six of the study isolates plus an additional eight S. aureus strains from vascular wound infections showed greater susceptibility of the strains to cefazolin than cefuroxime (median minimal inhibitory concentrations of 0.5 and 2.0 micrograms/ml, respectively, p less than 0.05). Furthermore, despite its more frequent intraoperative redosing, cefuroxime exhibited lower trough serum concentrations than cefazolin. Among cefuroxime recipients, infection-associated procedures were significantly longer than infection-free procedures (p less than 0.05), suggesting that low tissue antibiotic concentrations may have contributed to the pathogenesis of these infections. In contrast, the length of the procedure was not a risk factor for infection among cefazolin recipients.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation of in-vitro parameters of antimicrobial activity with prophylactic efficacy in an intradermal model of Staphylococcus aureus infection

Cephalosporins differ in their ability to prevent staphylococcal wound infection. Although the reasons for the observed differences are not fully understood, the susceptibility of cephalosporins to hydrolysis by staphylococcal beta-lactamase has been correlated with failures of prophylaxis. To investigate the effect of beta-lactamase stability and other in-vitro parameters of the bacterial-antimicrobial interaction on the efficacy of antimicrobial prophylaxis, two beta-lactamase-stable agents, cefuroxime and cefmetazole were compared to cefazolin and cefamandole in an in-vivo model of intradermal infection employing four strains of Staphylococcus aureus. Following intraperitoneal administration of a single dose of cephalosporin or placebo, guinea pigs were inoculated at multiple intradermal sites with 2 x 10(7) cfu of a strain of staphylococcus. For three strains, the area of induration at 24 h following inoculation was significantly smaller in guinea pigs receiving prophylaxis with cephalosporins versus placebo; no cephalosporin was effective against the fourth strain. Differences were also noted among the cephalosporins; prophylaxis with cefuroxime and cefmetazole resulted in smaller lesions than seen in animals given cefazolin or cefamandole. Poor correlation was noted between results of the in-vivo model and in-vitro determinants of the bacterial-antimicrobial interaction which were MIC values, time-kill curves, and the rates of beta-lactamase-mediated cephalosporin hydrolysis by the different strains. The model demonstrated unexplained failures of prophylaxis and unexpected differences in efficacy of various cephalosporins as has been described before. This study highlights the need for an improved animal model of surgical antimicrobial prophylaxis and the identification of in-vitro determinants that predict in-vivo prophylactic efficacy more accurately.

Purification of Staphylococcus aureus beta-lactamases by using sequential cation-exchange and affinity chromatography

Boronic acids are active-site inhibitors of serine beta-lactamases, and a phenylboronic acid-agarose affinity column has been used to purify beta-lactamase from crude cell extracts of several bacterial species. We applied phenylboronic acid-agarose chromatography to the purification of Staphylococcus aureus beta-lactamase. Two factors interfered with the success of the previously described single-step chromatographic protocol. First, staphylococcal beta-lactamase exhibited non-active-site-mediated adsorption to the agarose used as a support for the meta-aminophenylborate ligand, preventing the recovery of beta-lactamase from the column. Second, the staphylococcal beta-lactamases exhibited low affinity for meta-aminophenylborate with inhibition constants (Kis) ranging from 8.0 x 10(-3) to 20.0 x 10(-3) M. These problems were resolved by modifying the buffers utilized during chromatography and increasing the dimensions of the affinity column, and a two-stage procedure consisting of cation-exchange chromatography followed by affinity chromatography was used to purify each of the four variants of staphylococcal beta-lactamase. The mean specific activities of the purified type A, B, C, and D beta-lactamases were 44.6, 12.2, 10.6, and 30.8 mumol of nitrocefin hydrolyzed per min/mg of protein, respectively. Dimer formation, presumably from intramolecular cysteine-cysteine cross-linking, was observed with the type D beta-lactamase but not with the type A, B, or C enzyme.

Characterization of a widespread strain of methicillin-susceptible Staphylococcus aureus associated with nosocomial infections

Thirty Staphylococcus aureus isolates from five hospitals were determined to exhibit borderline susceptibility to the antistaphylococcal penicillins. Of the isolates submitted for phage typing, 96% belonged to phage group 94/96, and 96% possessed a common plasmid, pBW15. Also, four reference borderline-susceptible isolates from the Centers for Disease Control belonged to phage group 94/96 and possessed pBW15. Screening of 43 other phage group 94/96 isolates demonstrated that 36 (84%) contained pBW15 and exhibited the borderline phenotype. In contrast, pBW15 was not identified among 10 penicillin-susceptible, 10 methicillin-resistant, and 40 penicillin-resistant but non-borderline-susceptible S. aureus. These data show a close association between pBW15, phage group 94/96, and the borderline-susceptible phenotype. Furthermore, these isolates were associated with infections in multiple institutions, suggesting the widespread dissemination of a clinically important and pathogenic strain of S. aureus.

Use of extracts versus whole-cell bacterial suspensions in the identification of Staphylococcus aureus beta-lactamase variants

We previously have shown that extracts of S. aureus isolates which produce the recognized serotypes of staphylococcal beta-lactamase (A, B, C, D) differ in the rates at which they hydrolyze selected cephalosporins, exhibiting substrate profiles which are distinctive for each serotype. In an effort to simplify the methods employed in identifying the different staphylococcal beta-lactamases, we evaluated whether distinctive substrate profiles could be obtained by using whole-cell suspensions of 115 beta-lactamase-producing isolates of S. aureus. Compared with extracts from the same strains, the whole-cell bacterial suspensions not only were simpler to prepare but enabled beta-lactamase typing of a higher proportion of the evaluated strains (86 versus 97%, respectively). Furthermore, the use of whole-cell bacterial suspensions enabled the simultaneous quantitation of the beta-lactamase activity exhibited by each strain. Additionally, by comparing the quantitative activity of beta-lactamase-induced and -uninduced preparations of the same strain, induction ratios (i.e., induced/uninduced activity) could be derived, yielding information regarding the regulation of beta-lactamase production by each strain. We believe that the utilization of whole-cell methods, such as those employed in this study, will facilitate the investigation of qualitative and quantitative differences in beta-lactamase production among clinical and reference isolates of S. aureus.

Failure of cephalosporins to prevent Staphylococcus aureus surgical wound infections

Approximately 35,000 Staphylococcus aureus surgical wound infections occur annually in the United States. To investigate why S aureus causes infection despite the perioperative administration of cephalosporins, we compared 35 methicillin-susceptible isolates recovered from deep wound infections that complicated cefazolin prophylaxis (18 of 1650 patients) and cefamandole prophylaxis (17 of 3702 patients) with 64 colonizing isolates from presurgical patients. Compared with both colonizing and cefamandole-associated isolates, S aureus isolates from cefazolin-associated infections were more resistant to cefazolin by specialized assays. Staphylococcus aureus isolates that produced the A and C variants of staphylococcal beta-lactamase were associated with infections following cefazolin and cefamandole prophylaxis, respectively. These isolates hydrolyze the respective cephalosporins rapidly, suggesting that staphylococcal survival after perioperative prophylaxis may be mediated by in vivo degradation of the prophylactically administered cephalosporin. These data indicate that some S aureus wound infections occur because of deficiencies in antimicrobial effectiveness that are not detectable by routine susceptibility tests. This finding has important implications for the therapy and prevention of S aureus infection.

Selecting cost-effective antimicrobial prophylaxis in surgery: are we getting what we pay for?

The acquisition, administration, and infection-related expenses associated with the use of cefazolin or cefamandole during a prospective randomized comparison of antimicrobial prophylaxis in cardiac surgery were compared. Although the acquisition cost and administration charges related to the use of cefamandole were in excess of those incurred with cefazolin, differences in the charges associated with treating the wound infections that failed prophylaxis with each regimen made cefamandole the more cost-effective prophylactic agent (mean total of $766 per cefazolin recipient vs. $315 per cefamandole recipient). Analysis of prophylactic antimicrobials should include the expenses associated with "prophylactic failures." The agent that is least expensive for the pharmacy may not be the most cost-effective choice for the institution.

Evaluation of cefotaxime alone and in combination with desacetylcefotaxime against strains of Staphylococcus aureus that produce variants of staphylococcal beta-lactamase

We evaluated cefotaxime (CTX) alone and in combination with its metabolite, desacetylcefotaxime (dCTX) against strains of Staphylococcus aureus that produce the four recognized variants of staphylococcal beta-lactamase and a beta-lactamase-producing isolate characterized by the expression of borderline resistance to methicillin. Although macrodilution MICs revealed that dCTX was less active than CTX against these strains (geometric means of 16 micrograms/ml and 4 micrograms/ml, respectively), the addition of clinically achievable concentrations of dCTX to CTX resulted in a reduction in the observed CTX MICs. This effect was similar to although less pronounced than that obtained by combining clavulanic acid with cefazolin. The increased antistaphylococcal activity noted by MIC determinations was confirmed with kill-kinetic studies. Determination of the relative rates of hydrolysis of selected cephalosporins showed that neither CTX nor dCTX were appreciably hydrolyzed by the variant staphylococcal enzymes. Evaluation of the effect of CTX and dCTX upon the staphylococcal beta-lactamases demonstrated that neither agent inhibited the destruction of a 100 microM solution of nitrocefin, although the reduction of CTX and cefazolin MICs by low concentrations of dCTX suggests that the dCTX metabolite may act as a competitive inhibitor of beta-lactamase. These observations may explain the previously demonstrated clinical efficacy of CTX used alone for the treatment of serious infections caused by S. aureus.

Influence of preoperative showers on staphylococcal skin colonization: a comparative trial of antiseptic skin cleansers

We undertook a prospective randomized observer-blinded study comparing the ability of preoperative showers with chlorhexidine gluconate (Hibiclens), povidone-iodine (Betadine), and a lotion soap (Safe 'N Sure) to diminish the staphylococcal skin flora of patients. By block randomization, patients scheduled for an elective cardiac operation or coronary artery angioplasty were assigned to shower with one of the study skin cleansers either once (evening only) or twice (both evening and morning) before the procedure. Semiquantitative samples for culture were obtained from the subclavian and inguinal sites on the evening before the procedure (baseline culture) and again the next morning before the operation. The chlorhexidine skin cleanser consistently reduced staphylococcal colony counts at both the subclavian and inguinal sites before the procedure. This reduction was significant for patients showering both evening and morning (p less than 0.05). The use of the povidone-iodine skin cleanser inconsistently affected skin flora. Patients using lotion soap either experienced no change or had an increase in colony counts. Chlorhexidine is more effective than povidone-iodine in diminishing skin colonization with staphylococci in patients before operation. Repeated applications of chlorhexidine are superior to a single shower with this agent.

Efficacy of cefazolin, cefamandole, and gentamicin as prophylactic agents in cardiac surgery. Results of a prospective, randomized, double-blind trial in 1030 patients

In an effort to develop an improved regimen of antibiotic prophylaxis in cardiac surgery, 1030 patients who were to have elective cardiothoracic surgery involving a median sternotomy were selected at random to receive cefamandole or cefazolin, with or without gentamicin, in a prospective double-blind study. Cefazolin was significantly less effective than cefamandole at both the sternal (1.8% vs. 0.4%, respectively, p less than 0.05) and donor sites (1.3% vs. 0%, respectively, p less than 0.02). Seven Staphylococcus aureus infections occurred among cefazolin recipients as compared with no such infections among the patients receiving cefamandole (p less than 0.01). All five wound infections yielding fungi or gentamicin-resistant gram-negative rods occurred in patients who had received gentamicin as a second prophylactic agent. These data suggest that gentamicin has no role as a prophylactic antibiotic in cardiac surgery and that, compared with cefamandole, cefazolin offers unreliable prophylaxis against deep infection at both the sternal and donor sites.

Reexpansion pulmonary edema after pneumothorax

Spontaneous pneumothorax is a common disorder that is easily recognized and treated. Occasionally reexpansion of the collapsed lung is complicated by unilateral pulmonary edema, heralded by tachypnea, unilateral rales, and profuse expectoration of frothy secretions within several hours of reexpansion. Severe morbidity and death may result. Increased duration of pneumothorax and the use of suction are important factors in the generation of reexpansion pulmonary edema. Increased pulmonary capillary permeability rather than hydrostatic transudation is believed to underlie its development. For patients with pneumothorax of prolonged duration close observation in an intensive care unit for 24 hours after evacuation of air and the routine use of supplemental oxygen during and after lung reexpansion seem indicated.

Neurogenic bladder in Rocky Mountain spotted fever

A previously healthy 29-year-old man who had verified Rocky Mountain spotted fever had a flaccid neurogenic bladder that resolved as he recovered from his infection. The neurologic complications of this disease are presumed to be related to vasculitis with resulting ischemia or infarction of nervous tissue.