Antibacterial drug discovery: is it all downhill from here?

There has been a marked decline in the industrial research aimed at discovering novel antibacterial agents, including new drugs that target resistant organisms. While this decline may reflect past cyclical changes that often affect resource allocation at pharmaceutical companies, this decline is occurring at a time of increasing levels of antibacterial drug resistance and meagre pipelines of new agents that are active against them. There are multiple reasons for this decline, although few are unique to antibacterial drug discovery research. These include: lack of industry productivity, increasing size of clinical trials, increased generic competition and other pressures on drug pricing, a crowded and confused marketplace and industry consolidation. And while many (if not most) large companies and biotechs have exited the field or severely curtailed their research, others have made it a point to continue their efforts, citing both the unmet medical need and a large and apparently growing market. Despite the fact that some companies have remained engaged, the view here is that the current level of industrial effort is insufficient to sustain a healthy flow of new and better agents that are needed to counter the imminent threat of bacterial drug resistance. Therefore, a clear and urgent need for finding ways to improve the level and quality of industrial research in this area is apparent.

Mechanism of inhibition of the class A beta -lactamases PC1 and TEM-1 by tazobactam. Observation of reaction products by electrospray ionization mass spectrometry.

The reactions of class A beta-lactamases PC1 and TEM-1 with tazobactam (TZB), a potent penicillanic sulfone inhibitor for class A beta-lactamases, were studied using electrospray ionization mass spectrometry (ESI/MS). Following inactivation of the beta-lactamases by TZB, new abundant high mass components were observed including three with molecular masses of 52, 70, and 88 Da greater than PC1 and TEM-1, respectively, and a component with a molecular mass of 300 Da greater than PC1. In addition, three TZB reaction products with molecular masses of 248, 264, and 280 Da were observed. High performance liquid chromatography (HPLC)/ESI/MS analysis of the TZB-PC1 adduct digested with Glu-C revealed three new components with masses 52, 70, and 88 Da greater than that of the peptide composed of amino acid residues 58-82 and one new component with a mass 70 Da greater than that of the peptide composed of amino acid residues 125-141. HPLC/ESI/MS/MS analysis of the two digested peptides whose masses increased by 70 Da indicated that Ser-70 and Ser-130 were the most likely TZB-modified amino acid residues. Based on these data, a mechanism for the inactivation of the class A beta-lactamases by TZB is proposed. In this scheme, initial acylation of Ser-70 by TZB and opening of the lactam ring are followed by one of several different events: (1) the rapid decomposition of TZB with loss of the enamine moiety to form the propiolylated enzyme, (2) an intramolecular nucleophilic displacement of the imine or enamine moiety by Ser-130 to form a cross-linked vinyl ether, and (3) hydrolysis of the imine or enamines to form a Ser-70-linked aldehyde.

Respiratory syncytial virus infection: an emerging or unappreciated infection?

Respiratory syncytial virus (RSV) is a ubiquitous cause of respiratory infection with a worldwide distribution and seasonal occurrence. Natural immunity does not normally follow infection, and reinfection is the rule. Reinfection may even occur within a single season. Although its role in serious lower respiratory tract infection (LRTI) in infants is well recognized, the importance of RSV in disease of older children, adults and, especially in the elderly, is poorly appreciated. In large surveillance studies, RSV is at least as important in contributing to respiratory morbidity and mortality as influenza. These data would suggest that tens of millions of Americans suffer respiratory disease (LRTI and upper respiratory tract infection [URTI]) from RSV each winter. In addition, otitis media and episodic bronchospasm are complications following RSV infection. Unfortunately, there is no vaccine yet available for prevention. Prevention of disease with a hyperimmune globulin (RSVIG) and with a monoclonal antibody (MAb) (palivizuMab) is possible, but only for those neonates and infants at highest risk of morbidity and mortality from RSV infection. Therapy with the only approved antiviral for this indication, ribavirin, is difficult, is of questionable efficacy, and the compound itself is teratogenic. Infection by RSV constitutes an enormous but unappreciated medical need worldwide.

Class A beta-lactamases--enzyme-inhibitor interactions and resistance.

Beta-Lactamases of Ambler's Class A are the most commonly encountered mechanism of bacterial resistance to beta-lactam antibiotics. In the face of selective pressure arising from use of either newer cephalosporins or beta-lactam/beta-lactamase inhibitor combinations, mutations arose among Class A beta-lactamase genes, leading to resistance. Clavulanic acid, a naturally occurring clavam, and the penicillanic acid sulfones sulbactam and tazobactam are the inhibitors in clinical use. This review focuses on the mechanism of inhibition by these currently marketed beta-lactamase inhibitors and on the mechanism by which specific amino acid substitutions might lead to resistance. The key amino acid positions important for inhibitor-resistance include Met69, Ser130, Arg244, Arg275, and Asn276. Ser130 is vital to the chemical mechanism of inhibition. Arg244 appears to be coordinated to Arg275 and Asp276 by hydrogen bonds. Arg244 is involved in positioning beta-lactams, especially penicillins and beta-lactamase inhibitors, via their carboxyl groups. Site-directed mutagenesis studies confirm the role of Arg244 and its coordinating partners in beta-lactam turnover and in the reactions leading to enzyme inactivation. This mechanism is dependent on the donation of a proton via a water coordinated to Arg244 and Val216 to clavulanic acid to allow formation of a favorable leaving group. This proton donation is probably not required for formation of a favorable leaving group for the sulfone inhibitors sulbactam and tazobactam. Therefore, some amino acid substitutions have differing effects on inhibition by clavulanic acid compared with the penicillanic acid sulfones. Met69 may play a more structural role in beta-lactam positioning within the oxyanion hole.

Kinetic analysis of an inhibitor-resistant variant of the OHIO-1 beta-lactamase, an SHV-family class A enzyme.

The Met69-->Ile mutant of the OHIO-1 beta-lactamase, an SHV-family enzyme, is resistant to inactivation by beta-lactamase inhibitors. Analysis of purified Met69-->Ile enzyme reveals that its isoelectric point (pI 7.0) and CD spectrum are identical with those of the OHIO-1 enzyme. Levels of beta-lactamase expression in Escherichia coli as determined by immunoblotting are similar for OHIO-1 and Met69-->Ile beta-lactamase. The kinetic constants of the Met69-->Ile enzyme compared with OHIO-1 are smaller for benzylpenicillin (Km = 6 microM compared with 17 microM; kcat = 234 s-1 compared with 345 s-1 respectively) and carbenicillin (Km = 3 microM compared with 17 microM; kcat = 131 s-1 compared with 320 s-1 respectively). For the cephalosporins cephaloridine and 7-(thienyl- 2-acetamido)-3-[2-(4-N,N- dimethylaminophenylazo)pyridinium-methyl]-3-cephem-4-carboxylic acid (PADAC), a similar pattern is also seen (Km=38 microM compared with 96 microM and 6 microM compared with 75 microM respectively; kcat = 235 s-1 compared with 1023 s-1 and 9 s-1 compared with 50 s-1 respectively). Consistent with minimum inhibitory concentrations that show resistance to beta-lactam beta-lactamase inhibitors, the apparent Ki values, turnover numbers and partition ratios (kcat/kinact) for the mechanism-based inactivators clavulanate, sulbactam and tazobactam are increased. The inactivation rate constants (kinact) are decreased. The difference in activation energy, a measurement of altered affinity for the wild-type and mutant enzymes leading to acylation of the active site, reveals small energy differences of less than 8.4 kJ/mol. In total, these results suggest that the Met-->Ile substitution at position 69 in the OHIO-1 beta-lactamase alters the active site, primarily affecting the interactions with beta-lactamase inhibitors.

Adhesion of Staphylococcus epidermidis and transposon mutant strains to hydrophobic polyethylene.

Staphylococcus epidermidis capsular polysaccharide adhesin (PS/A) and slime were studied as possible mediators of bacterial adhesion to NHLBI polyethylene (PE) under dynamic flow. This putative interaction was examined by quantifying the adhesion of M187 (PS/A+, slime+) parent strain and isogenic transposon mutant strain sn3 (PS/A-, slime-) to polyethylene (PE) under a range of physiologic shear stress conditions in both phosphate-buffered saline (PBS) and 1% platelet poor plasma (PPP). No significant differences in adhesion were noted between the M187 and sn3 strains in either test medium. However, adhesion of both strains in 1% PPP was decreased 75-95% compared to adhesion in PBS. In PBS, adhesion was shear stress dependent from 0-15 dyne/cm2, after which adhesion was comparatively shear stress independent. Adhesion in 1% PPP was independent of shear stress. Epifluorescent imaging of both strains labeled for slime confirmed the presence of slime on the surface of M187 and suggested that PS/A and slime promote the formation of large aggregates, as aggregates were totally absent in the images of the sn3 strain. The results suggest that PS/A and slime do not mediate S. epidermidis adhesion to bare PE or PE with adsorbed plasma proteins, but may be necessary for intercellular adhesion, which is important for biofilm formation.

Risk factors associated with permanent access-site infections in chronic hemodialysis patients.

OBJECTIVE: To determine the epidemiological risk factors associated with permanent access-site (PAS) infection in a population of chronic hemodialysis patients. DESIGN: Retrospective cohort analysis. SETTING: Hemodialysis unit of a 400-bed Department of Veterans' Affairs hospital. RESULTS: A cohort of 94 males (1,316 patient months) was studied. Fifty-one PAS infections in 31 patients were observed. Six patients had two PAS infections, four patients had three infections, and two patients had four infections. Twenty-nine of the 31 patients with PAS infections were bacteremic at least once. Univariate analysis identified seven factors significantly associated with PAS infection in this population: location of PAS other than forearm, type of vascular access (polytetrafluoroethylene [PTFE] versus endogenous arteriovenous [AV] fistula), limited ambulatory status, residence in a nursing home, bacterial infection at a distant site, number of access-site revisions, and number of hospitalizations. In a logistic regression analysis, only graft type and number of PTFE graft revisions were associated independently with PAS infection. The odds ratio for PAS infection in PTFE grafts compared to endogenous AV fistula was 7.8; the odds ratio for PAS infection with each PTFE graft revision was 1.5. CONCLUSIONS: PAS infections were associated independently with the type of graft and the number of PTFE graft surgical revisions.

Society for Healthcare Epidemiology of America and Infectious Diseases Society of America Joint Committee on the Prevention of Antimicrobial Resistance: guidelines for the prevention of antimicrobial resistance in hospitals.

Antimicrobial resistance results in increased morbidity, mortality, and costs of health care. Prevention of the emergence of resistance and the dissemination of resistant microorganisms will reduce these adverse effects and their attendant costs. Appropriate antimicrobial stewardship that includes optimal selection, dose, and duration of treatment, as well as control of antibiotic use, will prevent or slow the emergence of resistance among microorganisms. A comprehensively applied infection control program will interdict the dissemination of resistant strains.

Construction and characterization of an OHIO-1 beta-lactamase bearing Met69Ile and Gly238Ser mutations.

Amino acid changes that influence activity and resistance to beta-lactams and beta-lactamase inhibitors were explored by constructing the Gly238Ser and Met69Ile-Gly238Ser mutants of the OHIO-1 beta-lactamase, a class A enzyme of the SHV family. The Km values of cefotaxime and ceftazidime for OHIO-1 and Met69Ile beta-lactamases were > or = 500 microM. The Km of cefotaxime for the Gly238Ser beta-lactamase was 26 microM, and that of ceftazidime was 105 microM. The Km of cefotaxime for the Met69Ile-Gly238Ser beta-lactamase was 292 microM, and that of ceftazidime was 392 microM. For the beta-lactamase inhibitors clavulanate and sulbactam, the apparent Ki values for the Met69Ile-Gly238Ser enzyme were 0.03 and 0.15 microM, respectively. Relative Vmax values indicate that the Met69Ile-Gly238Ser mutant of the OHIO-1 beta-lactamase possesses cephalosporinase activity similar to that of the Gly238Ser mutant but diminished penicillinase activity. In an Escherichia coli DH5alpha strain that possesses a Met69Ile beta-lactamase of the OHIO-1 family, the added Gly238Ser mutation resulted in a phenotype with qualities that confer resistance to expanded-spectrum cephalosporins and, to a lesser extent, beta-lactamase inhibitors.

OHIO-1 beta-lactamase mutants: Asp179Gly mutation confers resistance to ceftazidime.

The Asp179Gly mutant of the OHIO-1 beta-lactamase, an SHV enzyme, was constructed to investigate the effect of disruption of the omega loop on beta-lactamase activity in this class A enzyme. In Escherichia coli DH5 alpha the strain possessing the Asp179Gly mutation of the OHIO-1 beta-lactamase demonstrated increased susceptibility to all beta-lactams except ceftazidime and ceftriaxone. The minimum inhibitory concentrations for ceftazidime and ceftriaxone increased from 0.25 and 0.015 microgram/ml to 4.0 and .25 micrograms/ ml respectively. For ceftazidime, a substrate not hydrolyzed by the wild-type enzyme (K(m) > or = 500 microM), the K(m) of the Asp179Gly mutant beta-lactamase was measured to be 7 microM and the Vmax was 0.13 microM/min. The minimum inhibitory concentrations, K(m), and Vmax for all other beta-lactams decreased. Our analysis of this OHIO-1 beta-lactamase mutant suggests that disruption of the salt bridge in the omega loop by substitution of a glycine at position 179 markedly decreases the catalytic efficiency of the enzyme.

Society for Healthcare Epidemiology of America and Infectious Diseases Society of America Joint Committee on the Prevention of Antimicrobial Resistance: guidelines for the prevention of antimicrobial resistance in hospitals.

Antimicrobial resistance results in increased morbidity, mortality, and costs of health care. Prevention of the emergence of resistance and the dissemination of resistant microorganisms will reduce these adverse effects and their attendant costs. Appropriate antimicrobial stewardship that includes optimal selection, dose, and duration of treatment, as well as control of antibiotic use, will prevent or slow the emergence of resistance among microorganisms. A comprehensively applied infection control program will interdict the dissemination of resistant strains.

Tazobactam is a potent inactivator of selected inhibitor-resistant class A beta-lactamases.

The beta-lactam beta-lactamase inhibitor combinations (ampicillin/clavulanate, ampicillin/sulbactam, ticarcillin/clavulanate and piperacillin/tazobactam) were tested against selected inhibitor-resistant class A beta-lactamases of the TEM and OHIO-1 varieties. Minimum inhibitor concentrations (MIC) revealed that the Escherichia coli DH5 alpha transconjugate strains that possessed the OHIO-1 beta-lactamase, Met69Ile mutant of the OHIO-1 beta-lactamase, TEM-30 (Arg244Ser) and TEM-31 (Arg244Cys) beta-lactamase were most susceptible to piperacillin and piperacillin/tazobactam. Kinetic experiments with each beta-lactamase demonstrated that tazobactam was 10-25-fold more potent than clavulanate or sulbactam against TEM-30 and TEM-31 beta-lactamase. Tazobactam was also as effective as clavulanate against the Met69Ile mutant of the OHIO-1 beta-lactamase. Among the clinically available beta-lactams and beta-lactamase inhibitors, piperacillin/tazobactam appears to be the most potent combination against selected inhibitor-resistant strains of TEM and OHIO-1 beta-lactamase.

Peptidoglycan synthesis and structure in Staphylococcus haemolyticus expressing increasing levels of resistance to glycopeptide antibiotics.

The structures of cytoplasmic peptidoglycan precursor and mature peptidoglycan of an isogenic series of Staphylococcus haemolyticus strains expressing increasing levels of resistance to the glycopeptide antibiotics teicoplanin and vancomycin (MICs, 8 to 32 and 4 to 16 microg/ml, respectively) were determined. High-performance liquid chromatography, mass spectrometry, amino acid analysis, digestion by R39 D,D-carboxypeptidase, and N-terminal amino acid sequencing were utilized. UDP-muramyl-tetrapeptide-D-lactate constituted 1.7% of total cytoplasmic peptidoglycan precursors in the most resistant strain. It is not clear if this amount of depsipeptide precursor can account for the levels of resistance achieved by this strain. Detailed structural analysis of mature peptidoglycan, examined for the first time for this species, revealed that the peptidoglycan of these strains, like that of other staphylococci, is highly cross-linked and is composed of a lysine muropeptide acceptor containing a substitution at its epsilon-amino position of a glycine-containing cross bridge to the D-Ala 4 of the donor, with disaccharide-pentapeptide frequently serving as an acceptor for transpeptidation. The predominant cross bridges were found to be COOH-Gly-Gly-Ser-Gly-Gly-NH2 and COOH-Ala-Gly-Ser-Gly-Gly-NH2. Liquid chromatography-mass spectrometry analysis of the peptidoglycan of resistant strains revealed polymeric muropeptides bearing cross bridges containing an additional serine in place of glycine (probable structures, COOH-Gly-Ser-Ser-Gly-Gly-NH2 and COOH-Ala-Gly-Ser-Ser-Gly-NH2). Muropeptides bearing an additional serine in their cross bridges are estimated to account for 13.6% of peptidoglycan analyzed from resistant strains of S. haemolyticus. A soluble glycopeptide target (L-Ala-gamma-D-iso-glutamyl-L-Lys-D-Ala-D-Ala) was able to more effectively compete for vancomycin when assayed in the presence of resistant cells than when assayed in the presence of susceptible cells, suggesting that some of the resistance was directed towards the cooperativity of glycopeptide binding to its target. These results are consistent with a hypothesis that alterations at the level of the cross bridge might interfere with the binding of glycopeptide dimers and therefore with the cooperative binding of the antibiotic to its target in situ. Glycopeptide resistance in S. haemolyticus may be multifactorial.

Ceftazidime-resistant Klebsiella pneumoniae isolates recovered at the Cleveland Department of Veterans Affairs Medical Center.

The rate of ceftazidime resistance among Klebsiella pneumoniae isolates recovered from patients at the Cleveland Department of Veterans Affairs Medical Center increased from 6% in the first quarter of 1993 to 28% in the first quarter of 1994. The outbreak was hospitalwide, with the highest rates of resistance occurring on wards where ceftazidime was administered most frequently. Although many plasmid patterns were observed in the clinical isolates, molecular epidemiological analysis with use of pulsed field gel electrophoresis revealed substantial similarities between the strains; this finding suggested that most of the strains-if not all of them-were derived from the original clone. The addition of piperacillin/tazobactam to the hospital formulary and educational efforts focused on minimizing the administration of ceftazidime were associated with a marked decrease in the drug's use and a concomitant decrease in the percentage of ceftazidime-resistant isolates. We have not yet observed a significant rise in the rate of resistance to piperacillin/tazobactam among clinical isolates of K. pneumoniae.

Serogroup-specific epidemiology of Streptococcus pneumoniae: associations with age, sex, and geography in 7,000 episodes of invasive disease.

A study sample of 7,010 episodes of invasive Streptococcus pneumoniae disease was obtained by combining 13 existing datasets. Disease episodes due to each of 12 pneumococcal serogroups (1, 3-9, 14, 18, 19, and 23) were then compared with episodes in a constant internal control group to describe serogroup-specific variations in disease frequency by age, sex, and geographic origin. The results are presented as odds ratios (with 95% confidence intervals) derived by logistic regression, with adjustment for the major confounders, including dataset of origin. Variation in the male:female ratios between serogroups is small, suggesting that capsular characteristics are an unlikely explanation for the male preference of S. pneumoniae. Serogroups associated with higher nasopharyngeal prevalence (e.g., 19 and 24) are relatively more common in Europe and North American, while the invasive serotypes 1 and 5 are much more common in South America. The custom of reporting serogroup frequencies in two age groups, children and adults, conceals much of the variation in the age distributions across the whole span of life. The reduction of risk associated with serogroups 6, 14, 18, 19, and 23 beyond childhood follows different gradients, being most abrupt in serogroups 14 and most gradual in serogroup 18. The relative risk of disease with serotype 1 declines steadily throughout life, while with serotypes 3 and 8 it increases over middle age. Serogroups 7 and 23 are found unusually frequently in the third decade of life. Because of the wide differences in the epidemiology of individual serogroups of S. pneumoniae, it is questionable whether pneumococcal infection should continue to be classified as a single disease entity.

Molecular genetics of resistance to both ceftazidime and beta-lactam-beta-lactamase inhibitor combinations in Klebsiella pneumoniae and in vivo response to beta-lactam therapy.

The molecular basis of ceftazidime resistance in 2 isolates of Klebsiella pneumoniae was studied. The first (21300) expressed resistance to ceftazidime and piperacillin-tazobactam. The second (26139) expressed resistance to ceftazidime but remained susceptible to piperacillin-tazobactam. The 2 strains harbored similar large plasmids that hybridized to TEM- and SHV-related beta-lactamase genes. An Escherichia coli strain harboring the plasmid conferring resistance to both compounds (pLRM7) produced beta-lactamases of pI 5.9 (TEM-6) and pI 7.6 (SHV-1). E. coli harboring the other plasmid (pLRM8) expressed only the TEM enzyme because of insertion of IS15 within blaSHV-1. In vivo studies suggested that resistance to beta-lactam-beta-lactamase inhibitor combinations conferred by pLRM7 will be clinically important. Clinical resistance to both extended-spectrum cephalosporins and beta-lactam-beta-lactamase inhibitor combinations is achievable via the production of two enzymes, with only one possessing an extended spectrum of activity.

Decreased teicoplanin susceptibility of methicillin-resistant strains of Staphylococcus aureus.

Between February 1992 and February 1993, 12 patients were seen who were infected or colonized with methicillin-resistant strains of Staphylococcus aureus. The strains had decreased susceptibility to teicoplanin (MICs, 8-16 micrograms/mL). Field inversion gel electrophoresis showed the strains belonged to three related clones (A1, A2, and A3). The patients were though to have acquired the strains nosocomially. Consistent with results of laboratory studies of teicoplanin-resistant S. aureus, all but 1 strain expressed a 35-kDa membrane protein, and 9 strains expressed increased levels of penicillin-binding protein 2 complex. Six strains were isolated from patients treated with glycopeptides. These data suggest that nosocomial transmission of S. aureus with decreased teicoplanin susceptibility may occur during glycopeptide use and that such strains develop resistance by a mechanism associated with the appearance of a 35-kDa membrane protein. Surveillance is necessary to monitor for the potential selection of resistant clones that may be capable of dissemination.

Vancomycin resistance in the enterococcus. Relevance in pediatrics.

Enterococci are nosocomial pathogens intrinsically resistant to a variety of commonly used antimicrobial agents. The frequent use of antimicrobial agents such as cephalosporins has been associated with the increased isolation of enterococci in pediatric hospitals. In addition to their intrinsic resistance traits, the enterococci have rapidly accumulated a variety of acquired resistance determinants. Strains that are resistant to all currently available antibiotics are now being isolated from infected children. The threat of untreatable enterococcal infection and the possibility that vancomycin resistance may spread from the enterococci to the more virulent pneumococci or staphylococci argue for vigilant surveillance for resistant strains, isolation and barrier precautions for infected patients, increased research into the mechanisms of resistance, and a reinvigorated effort to identify new classes of antimicrobial agents.

Bacteremia due to vancomycin-dependent Enterococcus faecium.

A recipient of small-bowel and liver transplants developed recurrent fever and polymicrobial bacteremia due to multiply resistant Enterobacter cloacae and an inducible VanB strain of Enterococcus faecium while receiving therapy with amikacin, imipenem, and vancomycin. These organisms could not be subcultured onto blood agar but did grow around the vancomyin disk on a direct-susceptibility test plate. Additional testing confirmed the strain as E. faecium, which would not grow in the absence of vancomycin. Growth around a disk containing D-alanyl-D-alanine was demonstrated. Spontaneous vancomycin-independent revertants were obtained at a frequency of approximately 1 x 10(-6). Two classes of vancomycin-independent revertants were obtained: one that was constitutively vancomycin resistant and one that was nonconstitutively vancomycin resistant. We hypothesize that the normal D-ala ligase is not expressed in the vancomycin-dependent strain; thus survival of these strains is dependent on expression of the VanB ligase, which produces a depsipeptide precursor that is resistant to vancomycin binding. This is the second reported case involving a clinically important vancomycin-dependent enterococcal strain. Awareness of the existence of these strains is important, especially when clinical and microbiological data are consistent with infection due to a fastidious or nutritionally-deficient organism.