Effect of the concentrations of magnesium and calcium on the in-vitro susceptibility of Pseudomonas aeruginosa to gentamicin

The antimicrobial properties of C-reactive protein (CRP)

C-reactive protein, CRP, is a predominant pattern-recognition receptor (PRR) in the plasma of the horseshoe crab, which recognizes lipopolysaccharide (LPS). Native CRP2 has previously been shown to exhibit agglutination activity against the polysialic capsule of Escherichia coli K1 but its role in bacterial clearance is not well characterized. In this work, the antimicrobial activity of a recombinant CRP2 isoform (rCRP2) was tested against E. coli, Pseudomonas aeruginosa and Staphylococcus aureus. rCRP2 agglutinates bacteria and exhibits bactericidal activity against Gram-negative bacteria. In addition, the antimicrobial activity of rCRP2 is calcium-independent. GST pulldown experiments suggest that in the naïve physiological state, CRP2 interacts with hemocyanin, native CRPs, a 35-kDa plasma lectin and an as yet unidentified 40-kDa protein. This interaction was enhanced upon Pseudomonas infection. We propose that rCRP2 is a PRR with potent antimicrobial activity and its interacting partners contribute to effective bacterial clearance.

Beyond Agar: Gel Substrates with Improved Optical Clarity and Drug Efficiency and Reduced Autofluorescence for Microbial Growth Experiments

Agar, a seaweed extract, has been the standard support matrix for microbial experiments for over a century. Recent developments in high-throughput genetic screens have created a need to reevaluate the suitability of agar for use as colony support, as modern robotic printing systems now routinely spot thousands of colonies within the area of a single microtiter plate. Identifying optimal biophysical, biochemical, and biological properties of the gel support matrix in these extreme experimental conditions is instrumental to achieving the best possible reproducibility and sensitivity. Here we systematically evaluate a range of gelling agents by using the yeast Saccharomyces cerevisiae as a model microbe. We find that carrageenan and Phytagel have superior optical clarity and reduced autofluorescence, crucial for high-resolution imaging and fluorescent reporter screens. Nutrient choice and use of refined Noble agar or pure agarose reduce the effective dose of numerous selective drugs by >50%, potentially enabling large cost savings in genetic screens. Using thousands of mutant yeast strains to compare colony growth between substrates, we found no evidence of significant growth or nutrient biases between gel substrates, indicating that researchers could freely pick and choose the optimal gel for their respective application and experimental condition.

Effect of Ca+ on the photobactericidal efficacy of methylene blue and toluidine blue against gram-negative bacteria and the dye affinity for lipopolysaccharides

BACKGROUND AND OBJECTIVES

Methylene blue (MB) and toluidine blue (TB) form metachromatic complexes with lipopolysaccharides (LPS). The greater photobactericidal efficacy of TB may be explained by its affinity for LPS. This study aims to elucidate the difference in photobactericidal efficacies between the dyes using Ca(2+) as a competitor for dye-binding sites on the bacterial outer membrane.

STUDY DESIGN/MATERIALS AND METHODS

Fixed dye concentration solutions with gram-negative bacteria and increasing concentrations of CaCl(2) were exposed to red laser light. Bacterial survival and spectrophotometry were used to describe the effect of Ca(2+) on dye interaction with bacteria and LPS.

RESULTS

MB-mediated photokilling was inhibited more significantly than that of TB. CaCl(2) inhibited dye photobleaching and suppressed the metachromatic reaction between the dyes and LPS, in particular TB.

CONCLUSIONS

CaCl(2) inhibits bacterial photokilling by binding with LPS, as well as other anionic polymers including outer membrane proteins. LPS is chiefly involved in TB-mediated photokilling, whereas outer membrane proteins probably are more involved in MB-mediated photokilling.

Butirosin compared with gentamicin in vitro and in vivo

Butirosin (BTN) (P. W. K. Woo, G. L. Coffey, H. W. Dion, S. A. Fusari, and G. D. Senos, U. S. Patent 3,541,078, 1970) is a new aminoglycoside antibiotic notably active against opportunist bacterial species within Pseudomonas, Klebsiella, Enterobacter, Serratia, and Proteus. Numerous comparative tests were carried out with BTN and gentamicin (GTM) in vitro and in experimental infections in mice. BTN was more active in Mueller-Hinton broth than in agar, but its activity was lessened at acid pH or under anaerobiosis, as has been observed with other aminoglycosides. In standard agar diffusion tests, inhibition zones greater than 12 mm around 30-mug BTN disks generally denoted susceptibility, equivalent to minimal inhibitory concentrations [Formula: see text] 25 mug/ml. Cross-resistance between BTN and GTM occurred in a variable manner, with a number of bacterial strains strongly resistant to GTM being moderately susceptible to BTN. In mice, after a single subcutaneous injection, absorption of both antibiotics was rapid, with peak serum levels occurring in 15 min; this was followed by rapid elimination with estimated serum half-lives of about 20 min for each. After peroral administration of high doses in mice, there was no appreciable absorption of BTN. Several tests were carried out to compare BTN and GTM with respect to minimal inhibitory concentrations in vitro, acute subcutaneous median mouse protective doses, peak serum levels at such doses, and the therapeutic ratios derived from acute median protective and lethal doses. Although GTM usually proved to be more potent antibacterially on a weight basis, observations on BTN indicated a superior effectiveness in terms of therapeutic ratios.

The effect of carbon dioxide on susceptibility testing of azithromycin, clarithromycin and roxithromycin against clinical isolates of Streptococcus pneumoniae and Streptococcus pyogenes by broth microdilution and the Etest: Artemis Project-first-phase study

OBJECTIVE: To evaluate the effect of carbon dioxide on the susceptibility testing, using broth microdilution and the Etest (AB Biodisk, Solna, Sweden), of azithromycin, clarithromycin and roxithromycin against Streptococcus pneumoniae and Streptococcus pyogenes. METHODS: Fresh clinical isolates collected from 36 hospital laboratories in 12 countries were evaluated using the Etest in the presence of carbon dioxide. The isolates were retested under ambient conditions (absence of carbon dioxide) using broth microdilution and/or the Etest. RESULTS: Carbon dioxide falsely elevated azithromycin, clarithromycin and roxithromycin MIC90S for S. pneumoniae, determined by the Etest, approximately 12-fold. Also, the azithromycin MIC90 for S. pyogenes was increased fourfold; the effect was less marked for clarithromycin and roxithromycin. When isolates were retested in the absence of carbon dioxide, using the Etest or microdilution, susceptibilities to azithromycin were comparable to those to clarithromycin (S. pneumoniae, 93.4% versus 91.3%; S. pyogenes, 96.4% versus 95.8%). Both organisms were less susceptible to roxithromycin (S. pneumoniae, 71.3%; S. pyogenes, 85.7%). An internal standard control, consisting of 50 isolates each of S. pneumoniae, S. pyogenes and Haemophilus influenzae, confirmed that azithromycin susceptibility testing resulted in falsely elevated MICs. CONCLUSIONS: Carbon dioxide falsely elevated azithromycin MICs for S. pneumoniae and S. pyogenes, with an apparent reduction in susceptibility. When the in vitro activity of azithromycin and other macrolides against S. pneumoniae and S. pyogenes is being evaluated, awareness of the pH effect is essential.

Mechanism of uptake of deglucoteicoplanin amide derivatives across outer membranes of Escherichia coli and Pseudomonas aeruginosa

Teicoplanin is a glycopeptide antibiotic which is ineffective against gram-negative bacteria because of its inability to penetrate the outer membrane. Removal of the sugar residues and attachment of polyamines to carbon 63 yielded two dibasic deglucoteicoplanin amides, MDL 62,766 (766) and MDL 62,934 (934), with moderate MICs for Escherichia coli (2 to 4 micrograms/ml) and Pseudomonas aeruginosa (8 to 32 micrograms/ml) compared with those of the monobasic teicoplanin aglycone (16 and > 1,024 micrograms/ml, respectively). MICs were increased 16- to 32-fold by Mg2+ supplementation of Luria broth but not by Na+ supplementation at an equivalent ionic strength. Both 766 and 934 were capable of binding to P. aeruginosa lipopolysaccharide (LPS) at Mg(2+)-binding sites, as assessed by dansyl polymyxin displacement experiments. Furthermore, both compounds increased E. coli and P. aeruginosa outer membrane permeability to the hydrophobic fluorescent probe 1-N-phenylnaphthylamine (NPN), whereas the parent compounds teicoplanin aglycone and teicoplanin and the beta-lactam ceftazidime were totally ineffective. Addition of 1 mM Mg2+ blocked the increase in outer membrane permeability. Compound 766 had a lower MIC than 934 for both bacteria tested, bound to LPS with a higher affinity, and permeabilized outer membranes to NPN at lower concentrations. We propose that both deglucoteicoplanin amides exhibit increased gram-negative activity by virtue of their ability to access the self-promoted uptake pathway across the outer membrane.

In vitro antibacterial activities of the fluoroquinolones PD 117596, PD 124816, and PD 127391

Three new aminopyrrolidine-substituted fluorocyclopropyl quinolones--PD 117596, PD 124816, and PD 127391--were tested for in vitro antibacterial activity against 349 bacterial strains, which are primarily clinical isolates. The minimum inhibitory concentrations (MIC) in micrograms/ml required for greater than or equal to 90% of strains were 0.03-0.06 for staphylococci (26 strains); 0.06-0.25 for Streptococcus pyogenes, S. agalactiae, S. pneumoniae, and Enterococcus faecalis (80); less than or equal to 0.015 for Branhamella catarrhalis, Haemophilus influenzae, and Neisseria gonorrhoeae (42); 0.06 for Enterobacteriaceae (97); 0.125-0.25 for Acinetobacter spp. (14); 0.5 for Pseudomonas aeruginosa (20); 0.125-1.0 for Bacteroides fragilis (13); and 0.25-0.5 for anaerobic cocci (11). These activities were generally superior to that of ciprofloxacin, imipenem, ampicillin, penicillin G, oxacillin, cefazolin, ceftazidime, cefoxitin, cefsulodin, aztreonam, piperacillin, amikacin, spectinomycin, doxycycline, erythomycin, clindamycin, metronidazole, and vancomycin. The activities of the new quinolones were generally unchanged with light, 50% human serum, aerobic/anaerobic atmosphere, 5% sodium choate, cation supplementation, and 100-fold increased or decreased inoculum; as with other quinolones, potency was measurably diminished with decreasing pH (pH less than or equal to 6.0) and in 100% urine.

Wide variability in Pseudomonas aeruginosa aminoglycoside results among seven susceptibility testing procedures

Seven commonly used antimicrobial susceptibility testing methods were used to test the susceptibility of 150 isolates of Pseudomonas aeruginosa against gentamicin, tobramycin, amikacin, carbenicillin, and piperacillin. Results were compared with respect to the susceptibility characteristics of the population of isolates as defined by each method. Conventional methods included agar disk diffusion and agar dilution, carried out in accordance with current recommendations of the National Committee for Clinical Laboratory Standards, as well as broth microdilution testing with cation-supplemented Mueller-Hinton broth (CSMHB). Methods in which instrumentation was used for result determination included the Autobac I, Avantage, Sensititre Autoreader (using a breakpoint panel at 18 h of incubation), and Vitek (AMS-240, using the GNS susceptibility card). When necessary for comparison, MIC data were converted to categorical interpretations (susceptible, intermediate, and resistant). With respect to gentamicin, no significant differences were noted among the results of disk diffusion, broth microdilution, Sensititre Auto breakpoint, or Vitek methods which characterized 60 to 67% of isolates as susceptible, 16 to 22% as intermediate, and 13 to 17% as resistant. In contrast, agar dilution, Autobac, and Avantage, although yielding gentamicin results similar to those of one another, were each significantly different in result reporting from the other four methods above for gentamicin results, and they characterized the Pseudomonas population largely as susceptible (88 to 97%), with 0 to 6% intermediate and only 3% to 6% resistant. More isolates were characterized as being resistant to gentamicin in the Avantage test if an assay broth supplemented with increased amounts of calcium was used. Cation impregnation of Autobac disks did not appreciably change Autobac results. The geometric mean MIC of gentamicin was 4 micrograms/ml lower in the agar dilution method than in the CSMHB microdilution method, despite monitoring of the agar for cation content through performance disk diffusion testing with P. aeruginosa ATCC 27853. Tobramycin activity was greater than gentamicin activity, and susceptibility to tobramycin ranged from 89 to 97%, with few statistically significant differences noted among the seven methods studied. Differences in MIC distribution and geometric mean MIC between agar dilution and CSMHB microdilution testing were minimal and suggested less of a cation influence on tobramycin than gentamicin results. Although amikacin was also more active than gentamicin (83 to 99% of isolates were susceptible), differences in the amikacin results among methods tended to reflect the same trends in reporting as seen with gentamicin testing, with the exception that results of Avantage testing were similar to those of disk diffusion, CSMHB microdilution, Sensititre, and Vitek. A difference in geometric mean MIC of 5 micrograms/ml between CSMHB testing and agar dilution testing suggested the influence of divalent cations on amikacin results. Few highly significant differences were noted among methods when isolates were tested against carbenicillin and piperacillin, except that Avantage piperacillin results (66% susceptible) and Autobac piperacillin results (98% susceptible) were noticeably different from the percent piperacillin susceptibility (range, 85 to 92%) measured by the other methods. Method-dependent variability among aminoglycoside susceptibility results, particularly when testing gentamicin, prevents meaningful comparison of Pseudomonas susceptibility trends among hospitals when different methods are used and promotes confusion and frustration among clinical microbiologists and clinicians owing to the uncertainties of clinical meaning of these data.

Serum bactericidal test

The serum bactericidal test represents one of the few in vitro tests performed in the clinical microbiology laboratory that combines the interaction of the pathogen, the antimicrobial agent, and the patient. Although the use of such a test antedates the antimicrobial era, its performance, results, and interpretation have been subject to question and controversy. Much of the confusion concerning the serum bactericidal test can be avoided by an understanding of the various factors which influence bactericidal testing. In addition, the methodologic aspects of the serum bactericidal test have recently been addressed and should place this test on firmer ground. New information on the clinical utility of this test is becoming available; additional data are needed to establish more clearly the usefulness of the serum bactericidal test in specific infections. Such clinical trials from multiple centers will enable firmer recommendations for the future use of the serum bactericidal test.

In vitro activity of CI-934, a quinolone carboxylic acid active against gram-positive and -negative bacteria

CI-934 is a totally synthetic difluorinated quinolinecarboxylic acid with an ethyl-amino-methyl pyrrolidine side chain, which has broad-spectrum antibacterial activity, including particular potency directed against streptococci and staphylococci. The CI-934 MIC (micrograms per milliliter) for 90% of the strains tested was 0.4 (range, 0.2 to 0.8) for a group of streptococci (pneumococci, viridans streptococci, Streptococcus faecalis, and Lancefield groups A, B, and C), 0.2 (0.05 to 0.2) for staphylococci (including methicillin-resistant Staphylococcus aureus), 0.025 (less than or equal to 0.003 to 0.025) for Haemophilus influenzae and Neisseria gonorrhoeae, 1.6 (0.1 to 25) for Enterobacteriaceae, 25 (3.1 to 25) for Pseudomonas aeruginosa, and 1.6 (0.05 to 3.1) for non-Bacteroides anaerobe species. CI-934 was equally active in vitro against multi-drug-resistant and -sensitive isolates, and cross-resistance was not apparent. Potency increased with alkalinity and was somewhat lower in urine. CI-934 was bactericidal. Inhibitory activity was generally unaffected by anaerobiosis, light, changes in inoculum size or cation concentration, or addition of human serum or sodium cholate.

Evaluation of the automicrobic system for susceptibility testing of Pseudomonas aeruginosa to gentamicin, tobramycin, and amikacin

The AutoMicrobic system (AMS; Vitek Systems, Inc., Hazelwood, Mo.) was studied for its ability to produce accurate and precise MIC interpretations for Pseudomonas aeruginosa susceptibility to gentamicin, tobramycin, and amikacin. MICs were determined in parallel on 200 selected P. aeruginosa isolates by using the AMS discrete-integer MIC program AMS p12.ROB for interpretation of the AMS Gram-Negative General Susceptibility Urinary Card, and a reference small-integer broth microdilution test. Parallel AMS and broth microdilution MICs were also replicated for three selected strains of P. aeruginosa for which MICs were representative of the dilution test ranges. For the 200 P. aeruginosa isolates, mean AMS MICs were significantly larger than the reference test mean MICs, coefficients of variation were approximately double those of the reference test, and correlation coefficients were unacceptably low for each antimicrobial agent. MIC replication studies for the three selected P. aeruginosa strains showed comparable AMS and reference mean MICs in the lower portions of the dilution ranges, significantly higher AMS mean MICs in the upper portions, and mean coefficients of variation of 63 and 9.6%, respectively, for replicated AMS and reference MICs. These results indicate that the AMS, in its present stage of development, does not produce acceptable MIC measurements for P. aeruginosa susceptibility to gentamicin, tobramycin, and amikacin.

In vitro activity of newer beta-lactam agents in combination with amikacin against Pseudomonas aeruginosa, Klebsiella pneumoniae, and Serratia marcescens

The in vitro activity of cefoperazone, ceftazidime, ceftizoxime, moxalactam, and N-formimidoyl thienamycin was evaluated alone and in combination with amikacin to assess possible synergistic activity against isolates of amikacin-resistant Pseudomonas aeruginosa and multidrug-resistant Serratia marcescens and Klebsiella pneumoniae susceptible to amikacin (one S. marcescens isolate was also resistant to amikacin). The checkerboard agar dilution method was used. Ceftazidime and thienamycin followed by moxalactam and cefoperazone were the most active agents versus the P. aeruginosa alone and in combination testing. Ceftazidime, moxalactam, and thienamycin showed the greatest activity against S. marcescens, and all agents except cefoperazone were active against K. pneumoniae. The finding of synergy or partial synergy in combination testing was found in the majority with all three genera, including levels below the breakpoint for both amikacin and the beta-lactam agents. This wide in vitro activity indicates that clinical evaluation of these agents in treatment of multidrug-resistant infections is warranted.

Pseudomonas aeruginosa

P. aeruginosa is widely distributed in nature and in the hospital environment with a predilection for moist areas. Its inherent resistance to many antimicrobials and its ability to produce many enzymes contribute to its pathogenic potential as both a primary and a secondary cause of infection. It is easily grown and identified in the microbiology laboratory. However, susceptibility testing remains a problem. Currently, the best approach to treatment is an aminoglycoside and an antipseudomonal beta-lactam antimicrobial. Typing can differentiate strains, but should be reserved for specific epidemiologic problems.

Association between serum inhibitory and bactericidal concentrations and therapeutic outcome in bacterial endocarditis

Several recent reviews on the therapy of bacterial endocarditis have recommended that a serum inhibitory and/or bactericidal concentration (SIC/SBC) of 1:8 or more be achieved to ensure successful therapeutic outcome. We conducted a methodologic and statistical analysis of the available literature on endocarditis to determine the association between SIC/SBC titers of 1:8 or more and therapeutic outcome. We reviewed 17 studies published between 1948 and 1980 in which both SIC/SBC and therapeutic outcome were available. Factors that affect outcome, such as age, duration of symptoms, organism, and valve status, varied widely among the 226 patients. The methods used to measure SIC/SBC differed with respect to the time of obtaining the blood specimen relative to the antibiotic dose, size of the bacterial inoculum, type of broth, and definition of the bactericidal end-point. None of the 17 studies showed a significant association between SIC/SBC titers of 1:8 or more and survival or bacteriologic cure. Fifteen of the 17 also failed to demonstrate a significant association between SIC/SBC titers of 1:8 or more and medical cure. However, each of the studies that failed to demonstrate an association between SIC/SBC titers of 1:8 or more and improved therapeutic outcome had an insufficient sample size to confidently exclude a false-negative result. Analysis of the published data reveals insufficient evidence to demonstrate that SIC and SBC titers are of prognostic value in the therapy of patients with bacterial endocarditis.

Cation components of Mueller-Hinton agar affecting testing of Pseudomonas aeruginosa susceptibility to gentamicin

Seven lots of Mueller-Hinton agar were examined for calcium and magnesium contents and their distribution in pools or compartments. Gel disruption and centrifugation yielded the soluble cations, which varied from 9 to 113% of the total calcium and from 76 to 102% of the total magnesium. Throughout the experiments, a standardized disk diffusion test, using Pseudomonas aeruginosa (ATCC 27852) and a 10-mug gentamicin disk, served as an indicator for medium performance. Zone diameters correlated well with the sums of the soluble calcium and magnesium values in the different lots (r = -0.85). Ionized calcium, presumably the biologically active ion, was measured with a calcium-specific electrode. It represented only a fraction of the soluble calcium pool in three lots. Autoclaving resulted in shifts of the cations between the different pools. Addition of magnesium to one medium lot resulted in shifts of soluble and ionized calcium, indicating an interdependence of calcium and magnesium, and zone diameters correlated with soluble magnesium (r = -0.98), soluble calcium (r = -0.96), and ionized calcium (r = -0.96) in this experiment. Manipulation of one medium to match the performance of another showed that excess amounts of both ions were required to obtain similar performance. Satisfactory performance of an individual medium can be obtained by cation supplementation, but simple adjustment will not suffice for all media. The interaction of the other cation pool components must also be evaluated.

In vitro studies of piperacilin, a new semisynthetic penicillin

Piperacillin, a new semisynthetic penicillin, was compared with other semisynthetic penicillins, cephalosporins, and aminoglycosides by the agar dilution method against 3,600 isolates of facultative gram-negative bacilli, Bacteroides fragilis, and enterococci. At 64 mug/ml, piperacillin inhibited 90% of the isolates in each group of organisms tested except for Escherichia coli (83% inhibited by 64 mug/ml). Compared with carbenicillin, piperacillin had a 16-fold increase in activity by weight against Pseudomonas aeruginosa and the enterococcus, an 8-fold increase against Serratia marcescens, and a 4-fold increase against B. fragilis and Enterobacter species. Piperacillin was highly active against carbenicillin-resistant Klebsiella pneumoniae and inhibited many aminoglycoside-resistant organisms. Except for P. aeruginosa, the minimum bactericidal concentration of piperacillin was usually within one tube dilution of the minimum inhibitory concentration. Approximately one-third of the gram-negative bacilli were inhibited synergistically by piperacillin plus amikacin, but no synergy could be demonstrated against enterococci. Piperacillin's in vitro activity against gram-negative bacilli was similar to gentamicin's except that it also included B. fragilis, and piperacillin was decidedly superior to presently available penicillins against K. pneumoniae.

In vitro activity of netilmicin compared with gentamicin, tobramycin, amikacin, and kanamycin

The in vitro activity of netilmicin was compared with that of gentamicin, tobramycin, amikacin, and kanamycin against 636 strains of bacteria recently isolated from clinical sources. Gentamicin was the most active antibiotic, but netilmicin and tobramycin closely paralleled it. Netilmicin was generally four-to eightfold less active than gentamicin against Serratia and group A streptococci, and was twofold less active against Pseudomonas aeruginosa. When effects of inoculum size and concentration of divalent cations in the media were evaluated, netilmicin was shown to be similar to gentamicin in vitro. Minimum inhibitory concentrations for P. aeruginosa were increased as much as 18-fold when the Mg(2+) and Ca(2+) concentrations were increased to physiological levels in Mueller-Hinton broth.

Comparative activity of netilmicin, gentamicin, amikacin, and tobramycin against Pseudomonas aeruginosa and Enterobacteriaceae

Netilmicin (Sch 20569), a semisynthetic aminoglycoside antibiotic, was compared with gentamicin, tobramycin, and amikacin against 242 clinical isolates of Pseudomonas and Enterobacteriaceae. The minimum inhibitory concentration (MIC) was determined in both solid and liquid media. Netilmicin exhibited typical aminoglycoside properties, such as little effect of inoculum size on MIC, relatively small gap between MIC and minimum bactericidal concentration, and potentiation of anti-Pseudomonas activity in the presence of carbenicillin. Netilmicin provided no advantage in antimicrobial activity over gentamicin for either Pseudomonas or Enterobacteriaceae. Nearly complete cross-resistance to netilmicin was encountered with isolates resistant to gentamicin in either solid or liquid media. Netilmicin was less active than gentamicin against isolates of Pseudomonas and Providencia. Major discrepancies between MIC values determined in agar as opposed to those determined in broth were encountered for most isolates of Pseudomonas but also, depending upon antibiotic tested, for between 15 and 40% of isolates of Enterobacteriaceae. This new aminoglycoside agent will be useful clinically only if it is shown to be significantly less toxic than presently available analogues.

Assay of gentamicin in cerebrospinal fluid

A comparison of standard curves obtained from a conventional plate diffusion assay method revealed significant differences when gentamicin standards were made up in different media. Standards made up in distilled water resulted in a curve which differed from that of standards made up in pooled human cerebrospinal fluid by a factor of up to 4. When the assay medium was supplemented with 0-5% sodium chloride, the difference between the two standard curves was reduced to a factor of about 1-5. The curve obtained from standards made up in 150 mM sodium chloride/4-5 mM calcium chloride correlated well with that from standards made up in cerebrospinal fluid. There was no evidence of gentamicin being bound to protein in the cerebrospinal fluid.

In vitro comparison of netilmicin, a semisynthetic derivative of sisomicin, and four other aminoglycoside antibiotics

One hundred isolates of Pseudomonas and Enterobacteriaceae, of which 85 were chosen because of their resistance to gentamicin or amikacin, were tested for susceptibility to netilmicin (SCH 20569), a new semisynthetic derivative of sisomicin, and to four other aminoglycosides. Tests were performed in Mueller-Hinton agar and, with 43 of these isolates, also in Mueller-Hinton broth. Most isolates of Escherichia coli, Klebsiella, Enterobacter, Citrobacter, and Serratia that were gentamicin resistant proved to be susceptible to netilmicin and amikacin. Tests of representative isolates of this group showed that they owed their resistance to the production of aminoglycoside-adenylylating enzymes. Four isolates of Serratia, detected by their resistance to amikacin, were also highly resistant to netilmicin but were susceptible to gentamicin. These isolates produced aminoglycoside-acetylating enzymes. Gentamicin-resistant Proteus and Providencia were, in general, highly resistant to netilmicin but were susceptible to amikacin. These isolates also produced aminoglycoside-acetylating enzymes. Most gentamicin-resistant strains of Pseudomonas were resistant to netilmicin, either by enzymatic aminoglycoside modification or by other undefined mechanisms. Thus, like amikacin, netilmicin extends the aminoglycoside susceptibility pattern of Enterobacteriaceae to include gentamicin-resistant isolates that produce aminoglycoside-adenylylating enzymes. It is ineffective against strains, some of them susceptible to amikacin, gentamicin, or tobramycin, that produce aminoglycoside-acetylating enzymes.

Antimicrobial susceptibility testing

Considerable advances have been made in antimicrobial susceptibility testing. This review emphasizes the continued efforts toward standardization of methods for dilution and diffusion testing, particularly in the area of variation in medium performance, methods for detection of ampicillin resistant Haemophilus influenzae, and attempts to develop rapid automated systems for susceptibility testing. Susceptibility testing of anaerobes continues to be controversial from the standpoint of both the selection of methods and the application of susceptibility results to the clinical situation. It is expected that these and other problems in susceptibility testing will be solved by continued application of the investigative approaches that have brought us where we are today.

Problems in evaluating nitrofurantoin susceptibility tests for Klebsiella-Enterobacter

Tube dilution susceptibility tests in Trypticase soy broth showed that resistance to gentamicin (minimum bactericidal concentration ≧12.5 μg/ml) among hospital isolates of Pseudomonas aeruginosa increased from 13.9% in 1969 to 38.9% in 1972. Transfer of drug resistance to six aminoglycosides from one wild Pseudomonas strain to another was accomplished in recombination experiments. A carbenicillin-resistant, beta-lactamase-producing strain served as the recipient. The exconjugant was resistant not only to aminoglycosides, including amikacin, but also to all clinically employed antimicrobials. Aminoglycoside resistance in the exconjugant was cured by sodium dodecyl sulfate. This transferable aminoglycoside resistance was not mediated by adenylylation or, as judged by bioassay, by other antibiotic-inactivating or -modifying processes.

Effects of divalent cations on binding of aminoglycoside antibiotics to human serum proteins and to bacteria

Binding of gentamicin and related deoxystreptamine-containing aminoglycoside antibiotics to proteins in human serum can vary significantly with changes in experimental conditions. The concentrations of divalent cations are important variables, and binding increases progressively as the concentrations of calcium and magnesium decrease. Maximal binding of deoxystreptamine-containing aminoglycosides to human serum is approximately 70% in the absence of divalent cations. The binding of (3)H-labeled gentamicin to Pseudomonas aeruginosa increases and its bactericidal activity against P. aeruginosa is enhanced in the absence of divalent cations. In contrast, binding of (3)H-labeled gentamicin to Escherichia coli and bactericidal activity against E. coli do not vary significantly in the presence and in the absence of divalent cations. Interference with uptake of gentamicin provides a plausible explanation for the observation that the minimal inhibitory concentration of gentamicin for P. aeruginosa increases as the concentration of calcium or magnesium in bacteriological media increases. Although significant binding of deoxystreptamine-containing aminoglycosides to plasma proteins does not occur under normal physiological conditions in man, the possibility remains that variations in protein binding of these aminoglycosides might be significant under pathological conditions.

Activity of three aminoglycosides and two penicillins against four species of gram-negative bacilli

Three aminoglycoside antibiotics and two penicillins were compared for their in vitro activity against 60 isolates of Serratia, Pseudomonas, Proteus mirabilis, and indole-positive Proteus sp. Testing was done by the agar dilution method using Mueller-Hinton broth solidified with 1.5% agar. The activity of amikacin, aminodeoxybutirosin, and gentamicin against Proteus and Pseudomonas, as related to their peak blood levels, showed no significant differences. Amikacin was the most active against Serratia marcescens. Results using Mueller-Hinton media in broth dilution tests correlated with the agar dilution method except for Pseudomonas aeruginosa. The minimal inhibitory concentration for aminoglycosides in agar was considerably greater than the minimal inhibitory concentration in Mueller-Hinton broth, and the disparity was related to the higher divalent cation concentration of agar. BL-P1654 and carbenicillin were similar except that carbenicillin was much more active against indole-positive Proteus sp. Additionally, the ratio of bactericidal to bacteriostatic concentrations of BL-P1654 was considerably greater than for carbenicillin.

Interaction of purulent material with antibiotics used to treat Pseudomonas infections

To define factors contributing to the adverse prognosis of patients with gram-negative bacillemia and abscess formation, we studied the interaction between polymyxin B, colistin sulfate, gentamicin, or carbenicillin with purulent material. Carbenicillin activity was not significantly altered by incubation with pus. Equal volumes of antibiotic and purulent sediment decreased the effective concentration of polymyxin B, colistin sulfate, or gentamicin from 100 mug/ml to 3 to 6 mug/ml. One milliliter of purulent sediment bound more than 700 mug of gentamicin and 1,500 mug of polymyxin B or colistin sulfate. This effect occurred rapidly, proceeded at 4 and 37 C, was stable for 24 to 48 h, and was altered, but not abolished, by varying the pH of the solution. Antibiotic activity could be removed from pus by high concentrations of protamine sulfate, heparin, sodium chloride, or potassium chloride, suggesting binding rather than inactivation.

Pharmacology of amikacin in humans

Amikacin is a new aminoglycoside antibiotic which is active in vitro against most isolates of gram-negative bacilli. A dose of 300 mg/m(2) intramuscularly produced a highest mean serum concentration of 25.4 mug/ml with a mean serum concentration of 3.1 mug/ml at 8 h. The same dose intravenously produced a highest mean serum concentration of 52.4 mug/ml with a mean serum concentration of 2.1 mug/ml at 8 h. The mean urinary excretion during the first 6 h was 75 and 66%, respectively. When amikacin was administered at a dose of 150 mg/m(2) every 6 h, there was evidence of some drug accumulation. A loading dose of 150 mg/m(2) administered intravenously over 30 min followed by 200 mg/m(2) administered as a continuous infusion every 6 h maintained serum concentrations of 8 mug/ml. No major toxicity was observed with any of these drug regimens.

Medium containing trypan blue and antibiotics for the detection of Cryptococcus neoformans in clinical samples

A medium containing trypan blue, gentamicin, and chloramphenicol is introduced for the detection of Cryptococcus neoformans and Cryptococcus species from clinical samples. Ten recently isolated strains of Cryptococcus species as well as several clinical isolates of C. neoformans incorporated trypan blue and produced dark blue colonies on this mycological medium, whereas other common yeasts were light blue. The laboratory diagnosis of two cases of cryptococcosis was accomplished by the isolation of C. neoformans on the antibiotic-dye-containing medium. Compared to conventional media supporting large numbers of Pseudomonas aeruginosa and other gram-negative bacilli, the new medium was selective for yeasts. In one instance, the colonization of the respiratory tract by C. neoformans which led to fungemia was traced by the use of the antibiotic-dye medium. The antibiotic mixture, utilized herein, was more effective in suppressing bacteria contained in samples from patients than a medium containing cycloheximide and chloramphenicol.

Activity of five aminoglycoside antibiotics in vitro against gram-negative bacilli and Staphylococcus aureus

The in vitro susceptibility to BB-K8, butirosin, gentamicin, sisomicin, and tobramycin of seven groups of clinically significant gram-negative bacilli and Staphylococcus aureus was assessed by using the International Collaborative Study-World Health Organization criteria. The activity of gentamicin, sisomicin, and tobramycin generally paralleled each other. Sisomicin was the most potent compound by weight and usually demonstrated the most rapid rate of killing. BB-K8 and butirosin were less potent, but higher serum levels may be achieved with these agents. BB-K8 generally showed the greatest ratio between achieveable mean peak serum levels and concentrations needed to inhibit [Formula: see text] of each group of organisms tested. Additionally, BB-K8 was active against six of seven highly gentamicin-resistant strains. All of these antibiotics showed diminished activity at pH 6.4 but only gentamicin and sisomicin showed occasionally enhanced activity at pH 8.4.

Effect of assay medium on the antibacterial activity of certain penicillins and cephalosporins

It is well known that the composition of the assay medium greatly affects the antimicrobial activity of aminoglycoside antibiotics. A similar response has now been observed with certain penicillins and cephalosporins. In the case of these compounds, this effect is apparently governed by the chemical nature of the penicillin 6- and cephalosporin 7-side chains. In comparison with their activity in Nutrient Broth, the activity of some of the beta-lactam antibiotics that have a weakly basic or basic group in their side chain was reduced as much as 40-fold in one or more of the following media: Mueller-Hinton, Trypticase soy, antibiotic assay, and heart infusion broths. In contrast, the assay medium had no effect on the activity of those compounds possessing an acidic or a nonionizable function in their side chain. The extent to which medium influences the antibacterial activity was also dependent upon the assay method and the organism, the effect being more pronounced in broth dilution than in agar dilution tests and occurring more frequently with gram-negative than with gram-positive organisms.

In vitro activity of tobramycin and gentamicin

The in vitro antimicrobial activity of tobramycin and gentamicin was compared against 362 bacterial isolates. The minimal inhibitory concentration (MIC) of tobramycin was fourfold less than the MIC of gentamicin against most of 119 Pseudomonas organisms. Gentamicin and tobramycin had similar in vitro activity against Enterobacteriaceae and Staphylococcus aureus. Proteus rettgeri were commonly resistant to both tobramycin and gentamicin. The 10-mug tobramycin disc separated resistant (MIC >/=5 mug/ml) and susceptible (MIC <5 mug/ml) organisms in 359 of 362 tested. In disc diffusion testing, the tobramycin and gentamicin zone diameters were found to vary significantly with concentrations of magnesium ions in the media employed. The MIC of tobramycin varied with the size of the inoculum, and tobramycin was most effective at a neutral pH.