Activities of vancomycin and teicoplanin against penicillin-resistant pneumococci in vitro and in vivo and correlation to pharmacokinetic parameters in the mouse peritonitis model

The activities of glycopeptides against pneumococci were studied in vitro and in vivo. The MICs of two glycopeptides, vancomycin and teicoplanin, in different media against 10 strains of pneumococci with different susceptibilities to penicillin were determined. The MICs of teicoplanin were four times lower than those of vancomycin in Mueller-Hinton media supplemented with 5% blood, but the MICs were similar in mouse and human sera supplemented with 5% blood. The serum protein binding levels in mouse and human sera were 90% for teicoplanin in both and 25 and 35%, respectively, for vancomycin. The MICs for vancomycin and teicoplanin were only correlated in human serum (P < 0.001). The single doses giving protection to 50% of the animals in the mouse peritonitis model after a lethal challenge of pneumococci, the ED50s, were similar for vancomycin and teicoplanin, between 0.1 and 1 mg/kg of body weight for all 10 strains. The log ED50s were significantly correlated only to the log MICs of teicoplanin determined for mouse serum with 5% blood (P = 0.01) and to the log MICs of vancomycin determined by the E test (P = 0.03). Among the pharmacokinetic parameters analyzed at the ED50s, the most constant parameter was the time for which the drug concentration exceeded the MIC (T(>MIC)) when each drug was considered separately; however, when both drugs were considered together, the maximum concentration of drug in serum (Cmax) varied the least. This indicates that both these parameters are of importance for predicting the effect of the drugs. We conclude that the effect of glycopeptides was not influenced by the penicillin resistance of the pneumococci, either in vitro or in vivo, and that the superior activity of teicoplanin over that of vancomycin in vitro was abolished in vivo, an effect which probably was due to the high serum protein binding of teicoplanin. Both the pharmacokinetic parameters T(>MIC) and Cmax are important predicting the effect of glycopeptides, but the pharmacodynamics of glycopeptides are still not completely elucidated.     

Susceptibilities of penicillin- and erythromycin-susceptible and -resistant pneumococci to HMR 3647 (RU 66647), a new ketolide, compared with susceptibilities to 17 other agents

Susceptibility of 230 penicillin- and erythromycin-susceptible and -resistant pneumococci to HMR 3647 (RU 66647), a new ketolide, was tested by agar dilution, and results were compared with those of erythromycin, azithromycin, clarithromycin, roxithromycin, rokitamycin, clindamycin, pristinamycin, ciprofloxacin, sparfloxacin, trimethoprim-sulfamethoxazole, doxycycline, chloramphenicol, cefuroxime, ceftriaxone, imipenem, and vancomycin. HMR 3647 was very active against all strains tested, with MICs at which 90% of the strains were inhibited (MIC90s) of 0.03 microg/ml for erythromycin-susceptible strains (MICs, < or =0.25 microg/ml) and 0.25 microg/ml for erythromycin-resistant strains (MICs, > or =1.0 microg/ml). All other macrolides yielded MIC90s of 0.03 to 0.25 and >64.0 microg/ml for erythromycin-susceptible and -resistant strains, respectively. The MICs of clindamycin for 51 of 100 (51%) erythromycin-resistant strains were < or =0.125 microg/ml. The MICs of pristinamycin for all strains were < or =1.0 microg/ml. The MIC90s of ciprofloxacin and sparfloxacin were 4.0 and 0.5 microg/ml, respectively, and were unaffected by penicillin or erythromycin susceptibility. Vancomycin and imipenem inhibited all strains at < or =1.0 microg/ml. The MICs of cefuroxime and cefotaxime rose with those of penicillin G. The MICs of trimethoprim-sulfamethoxazole, doxycycline, and chloramphenicol were variable but were generally higher in penicillin- and erythromycin-resistant strains. HMR 3647 had the best kill kinetics of all macrolides tested against 11 erythromycin-susceptible and -resistant strains, with uniform bactericidal activity (99.9% killing) after 24 h at two times the MIC and 99% killing of all strains at two times the MIC after 12 h for all strains. Pristinamycin showed more rapid killing at 2 to 6 h, with 99.9% killing of 10 of 11 strains after 24 h at two times the MIC. Other macrolides showed significant activity, relative to the MIC, against erythromycin-susceptible strains only.     

Comparison of three commercial MIC systems, E test, fastidious antimicrobial susceptibility panel, and FOX fastidious panel, for confirmation of penicillin and cephalosporin resistance in Streptococcus pneumoniae

The performances of three commercial broth microdilution MIC assays adapted for use with fastidious organisms--the E test (ET), Fastidious Antimicrobial Susceptibility panel (FAS), and FOX Fastidious panel (FOX)--were compared with a MIC using Mueller-Hinton broth with 5% lysed horse blood (MHLHB) to confirm penicillin and cephalosporin resistance in clinical isolates of Streptococcus pneumoniae. Of the isolates screened for penicillin resistance, 5 (12.8%) were categorized as susceptible, 16 (41.0%) were categorized as intermediate, and 18 (46.2%) were categorized as resistant by MHLHB. Only the isolates exhibiting intermediate-to-resistant MICs were included in the comparison. Agreement within +/- 1 log2 dilution was found in 91, 21, and 76% of the ET, FAS, and FOX MICs, respectively, compared with the MHLHB MIC. No very major or major discrepancies occurred with the ET or FOX; however, two very major interpretive errors occurred with the FAS. Agreement between the ET and MHLHB for cefotaxime, ceftriaxone, and cefuroxime was 88, 85, and 100%, respectively. Less than 50% of cephalosporin MICs categorized as > 0.5 microgram/ml by MHLHB were detected by FAS or FOX. Of the methods compared, the ET was the most reliable alternative for susceptibility testing of pneumococci.     

MIC and time-kill study of antipneumococcal activities of RPR 106972 (a new oral streptogramin), RP 59500 (quinupristin-dalfopristin), pyostacine (RP 7293), penicillin G, cefotaxime, erythromycin, and clarithromycin against 10 penicillin-susceptible and -resistant pneumococci

Broth MICs and time-kill studies were used to test the activity of RP 59500 (quinupristin-dalfopristin), RPR 106972, pyostacine (RP 7293), erythromycin, clarithromycin, and cefotaxime for four penicillin-susceptible (MICs of 0.008 to 0.03 microgram/ml), two penicillin-intermediate (MIC of 0.25 microgram/ml), and four penicillin-resistant (MIC of 2.0 to 4.0 micrograms/ml) strains of pneumococci: 6 of 10 strains were resistant to macrolides (MICs of > or = 0.5 microgram/ml). MICs of RP 59500 (0.5 to 1.0 microgram/ml), RPR 106972 (0.125 to 0.25 microgram/ml), and pyostacine (0.125 to 0.25 microgram/ml) did not alter with the strain's penicillin or macrolide susceptibility status. Three penicillin-susceptible strains and one penicillin-intermediate strain were susceptible to macrolides (MICs of < or = 0.25 microgram/ml); the macrolide MICs for the remaining strains were > or = 4.0 micrograms/ml. Cefotaxime MICs rose with those of penicillin G, but all strains were inhibited at MICs of < or = 2.0 micrograms/ml. RP 59500 was bactericidal for all strains after 24 h at 2 x MIC and yielded 90% killing of all strains at 6 h at 2 x MIC; at 8 x MIC, RP 59500 showed 90% killing of six strains within 10 min (approximately 0.2 h). In comparison, RPR 106972 was bactericidal for 9 of 10 strains at 2 x MIC after 24 h and yielded 90% killing of all strains at 2 x MIC after 6 h; 90% killing of six strains was found at 8 x MIC at 0.2 h. Results for pyostacine were similar to those of RPR 106972. Erythromycin and clarithromycin were bactericidal for three of four macrolide-susceptible strains after 24 h at 4 x MIC. Clarithromycin yielded 90% killing of three strains at 8 x MIC after 12 h. Cefotaxime was bactericidal for all strains after 24 h at 4 x MIC, yielding 90% killing of all strains after 6 h at 4 x MIC. All three streptogramins yielded rapid killing of penicillin- and erythromycin-susceptible and -resistant pneumococci and were the only compounds which killed significant numbers of strains at 0.2 h.     

Predation of livestock ticks by chickens as a tick-control method in a resource-poor urban environment

The increasing prevalence of streptococci as causes of potentially fatal nosocomial bacteremia requires that antimicrobial agents used for empiric therapy in hospitalized patients include both pneumococci and viridans group streptococci as well as beta-hemolytic streptococci in their activity profile. In this study, the in vitro activity of cefepime, a new fourth-generation cephalosporin, was compared with other cephalosporins versus 197 nosocomial blood stream isolates of streptococci (20 Streptococcus pneumoniae, 104 viridans group, and 73 beta-hemolytic) isolated from patients at more than 30 medial centers from 1995 to 1997. Additional agents tested included penicillin, erythromycin, and vancomycin. Overall, cefepime inhibited 83% of the isolates at concentrations < or = 0.5 microgram/mL and 100% at < or = 8 micrograms/mL. By comparison, ceftazidime inhibited 35 and 88% of isolates at the same concentrations. Cefepime was approximately eightfold more potent than ceftazidime against S. pneumoniae, viridans group streptococci, and beta-hemolytic streptococci. Among the 42 isolates with penicillin MICs > 0.12 microgram/mL, 100% were inhibited by cefepime and only 48% by ceftazidime at < or = 8 micrograms/mL. The rank order of activity for all six agents against the 197 isolates was vancomycin > ceftriaxone > cefepime > penicillin > erythromycin > ceftazidime. Based on the results of the present study, cefepime and ceftriaxone were the superior cephalosporins in potency and spectrum for empiric coverage of patients at risk for streptococcal blood stream infections.     

Storage of trachea for X-ray microanalytical studies of tracheal submucosal glands

Previous studies have suggested that penicillin-resistant pneumococcal isolates (especially those with MIC > 1 microgram/mL) usually are clonally related. To test this hypothesis, the molecular epidemiology of 29 clinical isolates of penicillin-resistant pneumococci (of which 83% were also resistant to either cefotaxime or ceftriaxone) collected in central Taiwan was investigated by pulsed field gel electrophoresis. Twenty-seven distinct patterns were identified. Our results indicate that an increase in penicillin-resistant S. pneumoniae between April 1993 and June 1994 in central Taiwan is not due to the clonal dissemination of a limited number of epidemic strains.     

Antimicrobial resistance of 1,113 Streptococcus pneumoniae isolates from patients with respiratory tract infections in Spain: results of a 1-year (1996-1997) multicenter surveillance study. The Spanish Surveillance Group for Respiratory Pathogens

A nationwide susceptibility surveillance of 1,113 Streptococcus pneumoniae isolates was carried out and found the following percentages of resistance: cefuroxime, 46%; penicillin, 37%; macrolides, 33%; aminopenicillins, 24%; cefotaxime, 13%; and ceftriaxone, 8%. A significant (P < 0.05) seasonality pattern for beta-lactam antibiotics was observed. Resistance to macrolides was higher (P < 0.05) in middle-ear samples. Higher percentages of resistance to cefuroxime and macrolides were observed among penicillin-intermediate and -resistant strains, whereas high frequencies of resistance to aminopenicillins and expanded-spectrum cephalosporins were observed only among penicillin-resistant strains.     

Dysuria associated with urethral caruncle in the dog

Activity of CP 99,219 (trovafloxacin), clinafloxacin, ciprofloxacin, sparfloxacin, lomefloxacin and cefuroxime against 4 penicillin-susceptible, 2 penicillin-intermediate and 4 penicillin-resistant pneumococci was tested by MIC and time-kill methodology. Bacteriostatic values for all three groups did not differ significantly with all compounds tested except cefuroxime, and were lowest for trovafloxacin and clinafloxacin, followed by sparfloxacin, ciprofloxacin and lomefloxacin; cefuroxime yielded values which increased in line with those of penicillin G. The test compounds were bactericidal (i.e. they reduced original counts by > or = 3 log10 cfu/mL at one dilution above bacteriostatic levels) in most cases, though some strains showed slightly greater discrepancies between bacteriostatic and bactericidal levels of all compounds tested. Trovafloxacin, clinafloxacin and sparfloxacin yielded MIC and time-kill results which point to possible efficacy in treatment of penicillin-susceptible and -resistant pneumococcal infections.     

In vitro selection of resistance to four beta-lactams and azithromycin in Streptococcus pneumoniae

Selection of resistance to amoxicillin (with or without clavulanate), cefaclor, cefuroxime, and azithromycin among six penicillin G- and azithromycin-susceptible pneumococcal strains and among four strains with intermediate penicillin sensitivities (azithromycin MICs, 0.125 to 4 microg/ml) was studied by performing 50 sequential subcultures in medium with sub-MICs of these antimicrobial agents. For only one of the six penicillin-susceptible strains did subculturing in medium with amoxicillin (with or without clavulanate) lead to an increased MIC, with the MIC rising from 0.008 to 0.125 microg/ml. Five of the six penicillin-susceptible strains showed increased azithromycin MICs (0.5 to >256.0 microg/ml) after 17 to 45 subcultures. Subculturing in medium with cefaclor did not affect the cefaclor MICs of three strains but and led to increased cefaclor MICs (from 0.5 to 2.0 to 4.0 microg/ml) for three of the six strains, with MICs of other beta-lactams rising 1 to 3 twofold dilutions. Subculturing in cefuroxime led to increased cefuroxime MICs (from 0.03 to 0.06 microg/ml to 0.125 to 0.5 microg/ml) for all six strains without significantly altering the MICs of other beta-lactams, except for one strain, which developed an increased cefaclor MIC. Subculturing in azithromycin did not affect beta-lactam MICs. Subculturing of the four strains with decreased penicillin susceptibility in amoxicillin (with or without clavulanate) or cefuroxime did not select for beta-lactam resistance. Subculturing of one strain in cefaclor led to an increase in MIC from 0.5 to 2.0 microg/ml after 19 passages. In contrast to strains that were initially azithromycin susceptible, which required >10 subcultures for resistance selection, three of four strains with azithromycin MICs of 0.125 to 4.0 microg/ml showed increased MICs after 7 to 13 passages, with the MICs increasing to 16 to 32 microg/ml. All azithromycin-resistant strains were clarithromycin resistant. With the exception of strains that contained mefE at the onset, no strains that developed resistance to azithromycin contained ermB or mefE, genes that have been found in macrolide-resistant pneumococci obtained from clinic patients.     

Tissue water content in rats measured by desiccation

The in vitro activity of cefepime was compared to that of ceftazidime, ceftriaxone, and cefotaxime in a multicenter study involving 10 clinical microbiology laboratories and clinical isolates from 18 Brazilian hospitals from 7 cities (4 states). A total of 982 isolates consecutively collected between December 1995 and March 1996 were susceptibility tested by using Etest and following the NCCLS procedures for agar diffusion tests. The cefepime spectrum was broader than that of the other broad-spectrum cephalosporins against both Gram-negative rods and Gram-positive cocci. Cefepime was particularly more active against Enterobacter sp. (MIC90, 2 micrograms/ml), Serratia sp. (MIC90, 2 micrograms/ml) and oxacillin-susceptible Staphylococcus aureus (MIC90, 3 micrograms/ml). Against Pseudomonas aeruginosa, cefepime (MIC90, 16 micrograms/ml) was slightly more active than ceftazidime (MIC90, 32 micrograms/ml) and 8- to 16-fold more active than ceftriaxone of cefotaxime (MIC90, > 256 micrograms/ml). Our results show that nosocomial bacteria, especially Gram-negative rods, have a high rate of cephalosporin resistance in Brazil. However, part of these resistant bacteria remains susceptible to cefepime. The Etest was shown to be an excellent method for multicenter studies of the in vitro evaluation of new antimicrobial agents.     

Antipneumococcal activities of RP 59500 (quinupristin-dalfopristin), penicillin G, erythromycin, and sparfloxacin determined by MIC and rapid time-kill methodologies

Previous time-kill studies have shown that RP 59500 is rapidly bactericidal against pneumococci. To extend these findings, the activities of RP 59500, its two components RP 57669 RP 54476, penicillin G, erythromycin and sparfloxacin against 26 penicillin-susceptible, 25 penicillin-intermediate, and 25 penicillin-intermediate, and 25 penicillin-resistant pneumococci were determined by the agar dilution MIC and the time-kill testing methodologies within 10 min (ca. 0.2 h) and at 1 and 2 h. Respective agar dilution MICs at which 90% of isolates are inhibited for penicillin-susceptible, -intermediate, and -resistant strains were as follows: penicillin G, 0.03, 1, and 4 micrograms/ml;RP 59500, 1, 1, and 1 microgram/ml; RP 57669, 8, 32, and 16 micrograms/ml; RP 54476, > 128, > 128, and > 128 micrograms/ml; erythromycin, 0.06, 2, and > 128 micrograms/ml; and sparfloxacin, 1, 0.5, and 0.5 microgram/ml. RP 59500 was equally active (MIC at which 90% of isolates are inhibited, 1.0 microgram/ml) against erythromycin-susceptible and -resistant strains. Time-kill testing results showed that only RP 59500 at one to four times the MIC killed pneumococci at 0.2 h; RP 59500 was also the most active compound at 1 and 2 h. By comparison, penicillin and sparfloxacin at one, two, and four times the MICs reduced the original inoculum by > or = 1 log at 2 h for 46, 80, and 95% and for 50, 72, and 86% of strains, respectively. The killing activity of RP 59500 was the same against erythromycin-susceptible and -resistant strains. RP 57669, RP 54479, and erythromycin were either inactive or bacteriostatic at 2 h. Of all drugs tested, RP 59500 yielded the most rapid killing.     

Visual motion aftereffects: differential adaptation and test stimulation

Cefditoren (formerly ME-1206), a new orally administered cephalosporin, was evaluated in vitro against 1249 recently isolated strains of Streptococcus pneumoniae (500 strains), Moraxella catarrhalis (250 strains), and Haemophilus influenzae (499 strains). Reference National Committee for Clinical Laboratory Standards methods were used and the strains were representative for the current rates of beta-lactamase production or penicillin resistance. Cefditoren had MIC50/MIC90 results for Moraxella catarrhalis and Haemophilus influenzae of 0.12/0.5 and < or = 0.008/0.015 microgram/mL, respectively. The pneumococci were consistently twofold to eightfold more susceptible to cefditoren than other oral cephalosporins or penicillins. The MIC90 for penicillin-resistant S. pneumoniae was only 2 micrograms cefditoren/mL, and the highest recorded MIC was 4 micrograms/mL. Cefditoren appears to be a very promising beta-lactam possessing the greatest potency and potential spectrum versus contemporary (1997) respiratory tract pathogens.     

BL-S786, a new parenteral cephalosporin. II. In vitro antimicrobial activity comparison with six related cephalosporins

BL-S786 was compared by in vitro studies with 6 other parenteral cephalosporins (cefamandole, cefazolin, cefoxitin, cephaloridine, cephalothin and cephradine). The following parameters were assessed: Comparative MICs against a wide variety of bacterial isolates, MIC/MBC comparisons and the effect of inoculum size on the MIC. BL-S786 showed the greatest antimicrobial activity against K. pneumoniae, C. diversus and Salmonella species; was equal to cefamandole against E. coli, E. agglomerans and P. mirabilis; and was second to cefamandole against Shigella, E. tarda, C. freundii, E. cloacae, E. aerogenes and the pathogenic Neisseriae. Essentially no activity against Serratia and Pseudomonas species was observed. Compared to the other cephalosporins tested BL-S786 showed poor activity against staphylococci and streptococci. For most species tested, the MBC of the various cephalosporins was the same or within one dilution of their respective MICs. However, for Enterobacter and indole-positive Proteus species, the MBC of BL-S786 and cefamandole was usually larger than or equal to 8-fold higher than the MICs. Cefoxitin, on the other hand, showed little MIC/MBC variations against indole-positive Proteus species. Inoculum size had only a small effect on the MICs against most gram-negative species--in some instances greater than 64-fold increases in MIC resulted by increasing inoculum size from 10(5) to 10(7) organisms per ml.     

Activities of RPR 106972 (a new oral streptogramin), cefditoren (a new oral cephalosporin), two new oxazolidinones (U-100592 and U-100766), and other oral and parenteral agents against 203 penicillin-susceptible and -resistant pneumococci

Agar dilution was used to determine the MICs of RPR 106972 (a new oral streptogramin), cefditoren (a new oral cephalosporin), two new oxazolidinones (U-100592 and U-100766), and other oral and parenteral agents for 203 penicillin-susceptible and -resistant pneumococci. All pneumococci were inhibited by RPR 106972 at < or = 0.5 microgram/ml. Cefditoren was very active against all pneumococcal groups, with MICs of < or = 2.0 micrograms/ml. Amoxicillin with or without clavulanate was the next most active oral beta-lactam, followed by cefdinir, cefuroxime, cefpodoxime, and cefprozil. U-100592 and U-100766 were very active against all classes of pneumococci, with all MICs < or = 1.0 microgram/ml.     

Comparison of the antibacterial activity of nine cephalosporins against Enterobacteriaceae and nonfermentative gram-negative bacilli

The in vitro antibacterial activity of nine cephalosporins (cephalothin, cephaloridine, cephalexin, cefazolin, cefamandole, cefuroxime, cefatrizine, cefoxitin, and cefazaflur) was determined against 344 strains of Enterobacteriaceae and 99 nonfermentative gram-negative bacilli. Cefamandole, cefazaflur, and cefuroxime were the most active cephalosporins against the Enterobacteriaceae (with the exception of Serratia marcescens). However, cefoxitin was the only cephalosporin that inhibited all 30 S. marcescens strains in a concentration of 16 mug/ml and was by far the most active compound against selected cephalothin-resistant strains of Escherichia coli, Klebsiella, and Proteus mirabilis. Acinetobacter spp. were inhibited best by cefuroxime, but none of the cephalosporins had appreciable activity against the Pseudomonas spp.     

Strategies in the treatment of penicillin-resistant Streptococcus pneumoniae

The epidemiology, resistance mechanisms, susceptibility testing, treatment, prevention, and clinical importance of penicillin-resistant Streptococcus pneumoniae (PRSP) infection are discussed. PRSP is an established presence in the United States, with some geographic areas reporting decreased susceptibility in up to half of isolates. The mechanism of resistance to beta-lactam antibiotics in S. pneumoniae is genetic changes resulting in decreased binding of drug to the bacterial cell wall. Emerging PRSP strains have necessitated testing as a tool in selecting drugs for treating life-threatening infections. Opinions differ on how to treat these infections empirically. Non-life-threatening infections, such as otitis media, are still often treated successfully with amoxicillin, amoxicillin-clavulanate potassium, or a third-generation cephalosporin. Currently recommended initial treatment of pneumococcal pneumonia in otherwise healthy patients requiring hospitalization consists of cefuroxime, ceftriaxone, or cefotaxime; some authors continue to emphasize injectable penicillin. Once the mainstay of empirical treatment of pneumococcal meningitis, penicillin has largely been abandoned in favor of cefotaxime or ceftriaxone. Vaccination remains an underutilized strategy in atrisk populations. The clinical importance of penicillin resistance among pneumococci is still uncertain. Changing patterns in the susceptibility of S. pneumoniae to penicillin make selection of appropriate therapy increasingly difficult. Key considerations are the site of infection and the level of resistance.     

Primary and secondary stroke prevention in atrial fibrillation

The correlation between pharmacokinetic parameters and the in-vivo effect of antibiotics in relation to bacterial growth phases was evaluated using the mouse peritonitis model with a penicillin-resistant pneumococcus. Different 8 h dosing regimens were applied, with different total doses and initiated at different times during the bacterial growth phase. The effect was measured as the decline in bacterial counts in the peritoneal cavity. The pharmacokinetic parameters showed major changes during the phases of growth, as the serum elimination of penicillin decreased during the infection. The same effect of dosing regimens was observed in the exponential and stationary phases. In two regimens where T(>MIC) (the time the serum concentration exceeded the MIC) was 50% of the treatment period, a significantly better effect was achieved with a 2 hourly regimen than with a regimen with treatments every 20 min. The T(>MIC) of each dose was shown to be a critical parameter for achieving an effect in all growth phases. The maximum effect of penicillin, a 5-6 x log10 decline in bacterial counts in the peritoneum of the mice, was achieved when T(>MIC) was >50% of the treatment time or longer than approximately 40 min of each dose. The 50% effective dose for protection after a single injection, ED50, was measured in the different phases of the infection and found to increase with the duration of the pneumococcal infection, while mice treated 24 h after challenge were beyond therapeutic range. The correlation between the effect of penicillin and pharmacokinetic parameters appears to follow the same rules during the different in-vivo growth phases of pneumococci.     

Surrogate disks for predicting cefotaxime and ceftriaxone susceptibilities of Streptococcus pneumoniae

Cefotaxime- and ceftriaxone-resistant Streptococcus pneumoniae is now appearing in some medical centers, but 30-micrograms cefotaxime or 30-micrograms ceftriaxone disks are not reliable for detecting such strains. Studies were undertaken to select another cephalosporin disk that might be used as a screening test that could be used in conjunction with a 1-micrograms oxacillin disk. A 30-micrograms cefuroxime disk is proposed: strains with zones > or = 28 mm in diameter are predictably susceptible to cefotaxime and ceftriaxone, and those with smaller zones should be further studied to confirm resistance to either drug. A 30-micrograms ceftizoxime disk may also be used as a screening test with zones > or = 26 mm indicating susceptibility, but cefuroxime disks are preferred.     

Prevalence of outer membrane porin alteration in beta-lactam-antibiotic-resistant Enterobacter aerogenes

We evaluated the prevalence of impermeability as a mechanism associated with resistance against beta-lactam antibiotics in members of the family Enterobacteriaceae. During a 1-year period, 80 strains were selected from 3,110 routinely isolated strains according to their noticeable cross-resistance pattern to cephalosporins. They were tested for (i) outer membrane nonspecific porins involved in the entry of small hydrophilic molecules; (ii) the MICs of cefepime, cefotaxime, imipenem, and moxalactam; and (iii) beta-lactamase production. Immunological investigations using specific probes showed that 23 of 80 strains presented an alteration of the porin content, most of them expressing an additional resistance mechanism. The prevalence of this porin-deficient phenotype is especially high in Enterobacter aerogenes and concerns 6.4% of the clinical isolates.     

Structural specificity requirements in the binding of beta lactam antibiotics to human serum albumin

The binding of some cephalosporins of pharmacological interest, to human serum albumin was studied using ultrafiltration method. The identification of the binding sites in albumin was also performed using probes for the so-called sites I, II, bilirubin and fatty acids binding sites. Cephalosporins were classified into three groups according to their affinity for albumin: low affinity (K = 10-10(2) M-1), medium affinity (K = 10(3) M-1) and high affinity (K = 10(4) M-1). Cephalosporin binding to albumin produced a perturbation of several basic amino acids of the protein such as histidine and lysine. It was found that only cefuroxime, ceftazidime and cefoperazone interact slightly with site I on serum albumin, while site II possesses capacity to bind: cephradine, cephalexin, ceftazidime, ceftriaxone, cefoperazone, cefaclor and cefsulodin. The bilirubin binding site showed capacity to interact with a great number of cephalosporins: ceftriaxone, cefazolin, cephaloglycin, cefamandole, cefotaxime, cefoxitin, cefuroxime, cefoperazone and cefadroxil. Ceftriaxone showed capacity to bind to the fatty acid binding site on HSA. No relation was found between the displacement of the marker and the chemical nature of the substituents at R1 and R2. Cephalosporins interact with HSA at the binding region that involves: tyrosyl 411, histidyl 146 and lysyls 195, 199, 225, 240 and 525 residues. The chemical modification of specific amino acids showed that the interaction of these amino acids with beta lactam antibiotics is not carried out to the same extent for all the cephalosporins tested. The results obtained revealed that the binding sites for cephalosporins on albumin are structurally heterogeneous, having different amino acids in the vicinity of the ligand molecule.