Activity of netilmicin compared with those of gentamicin and tobramycin against enterobacteria and Pseudomonas aeruginosa

The inhibitory activity of netilmicin against 500 isolates of gram-negative bacteria was compared with those of gentamicin and tobramycin. Netilmicin was considerably less active than tobramycin and slightly less inhibitory than gentamicin for Pseudomonas aeruginosa but was at least as active against Escherichia coli and Klebsiella pneumoniae as were the other two antibiotics. A few Klebsiella and Serratia isolates resistant to gentamicin and tobramycin were inhibited by netilmicin. All three antibiotics were strongly bactericidal for E. coli, K. pneumoniae and P. aeruginosa but had less lethal activity against the otherwise susceptible Serratia isolates tested. Some necessary precautions in reading minimal inhibitory concentrations on agar media are stressed, and some possible advantages of a 4-h bactericidal test, using a constant antibiotic concentration, are defined.

Antibiotic resistance transfer from nonpathogenic to pathogenic bacteria

Alcaligenes species is a common contaminant of "wet" environmental areas on the surgical ward. Although thought to be a nonpathogenic organism, recent clinical experience on the burn and trauma service has led us to believe that antibiotic resistance transfer may occur between Alcaligenes and Pseudomonas aeruginosa. To evaluate this possibility, germ-free mice were contaminated with Alcaligenes species, which quickly established in the animals' gastrointestinal tracts. These animals then were burned and the wound was seeded with additional Alcaligenes. After 72 hours the average bacterial count was 4.5 X 10(6) cells/gm of tissue, and all animals survived. Ten additional germ-free mice were contaminated with a resistant (Amikacin, tobramycin, gentamicin, and Sisomicin) Alcaligenes species. When a Pseudomonas aeruginosa strain sensitive to these antibiotics was introduced into the environment, it rapidly overgrew the Alcaligenes species but developed resistance to those four antibiotics to which it had been sensitive previously. These animals were then subjected to a 10 second immersion burn, and the wound was seeded with the same strain of Alcaligenes. The Pseudomonas quickly overgrew the Alcaligens on the burn wound and became established, with an average count being 5.2 X 10(8) cells/gm of tissue. When this experiment was repeated, establishing antibiotic sensitive Pseudomonas in the germ-free animals prior to inoculation of resistant Alcaligenes, the R-transfer again occurred but required a longer time.

Cross infection in a surgical ward caused by Pseudomonas aeruginosa with transferable resistance to gentamicin and tobramycin

An outbreak of gentamicin- and tobramycin-resistant Pseudomonas aeruginosa infection occurred in a surgical ward over a three-month period. Resistant Ps. aeruginosa strains with the same serological, phage, and pyocin type were cultured from the urine of six patients. Identical organisms were found on urine bottles, bedpans, and the hands of attendant staff. Inadequate disinfection played a major role in cross-infection. Isolates of the epidemic strain from each of the patients and of an unrelated but similarly resistant Ps. aeruginosa from one of them could transfer resistance to a recipient strain of Ps. aeruginosa. Resistance to gentamicin, kanamycin, tobramycin, sulphonamides, and mercuric chloride was determined by R factors belonging to Pseudomonas incompatibility group P-3. Aminoglycoside resistance was due to acetylation.

Bacterial sensitivity to the aminoglycosides

From the reports of bacterial sensitivity testing reviewed here, it can be concluded that gentamicin has currently a very broad spectrum, with few resistant organisms. Gentamicin, amikacin, and tobramycin are the most effective of the aminoglycosides against Pseudomonas. They are also effective against Escherichia, salmonellae, staphylococci, pasteurellae, Klebsiella, and others; they are less effective against streptococci. Streptomycin is effective but bacterial resistance to it develops very rapidly and there are a large number of resistant strains involved in animal diseases. It is the least effective aminoglycoside that is commercially available. Neomycin is fairly effective against staphylococci, less effective against streptococci, and fairly effective against gram-negative intestinal organisms.

A simple method for determining bacterial susceptibility to aminoglycosides

A method is described of determining the susceptibility of enterobacteria to four amino-glycosides by means of an antibiotic diffusion method. The results are exose obtained by a method that involved titration in agar plates and the use of a multi-point inoculator. A statistical analysis showed that the diffusion method gave accurate reproducible results. The antibiotic can be incorporated into paper disks, paper strips or agar ditches and the calculation converting the measured zones of inhibition to MIC values is easily performed. The appropriate equation is derived from another--obtained from observations on a single strain of known sensitivity--describing the diffusion of the antibiotic from the reservoir.

Resistance to gentamicin, tobramycin and amikacin among clinical isolates of bacteria

Susceptibility to the administration of gentamicin, tobramycin and amikacin was determined for all isolates of aerobic and facultative gram-negative bacilli submitted for testing to the clinical bacteriology laboratory of the Massachusetts General Hospital between July 1, 1974, and June 30, 1976. In this 24-month period more than 46,000 isolates of bacteria were tested by the single-disc diffusion (Bauer-Kirby) method. Resistance to one or more of the aforementioned aminoglycosidic aminocyclitol antibiotics was found among 4,114 stains. Correlation with quantitative susceptibility test methods revealed that disc-diffusion methods using 10 microng discs accurately predicted resistance to gentamicin and tobramycin, but overestimated the prevalence of resistance to amikacin by 20 to 60%. Most of the gentamicin-resistant Enterobacteriaceae in this study were also cross-resistant to tobramycin but were susceptible to amikacin. Many gentamicin-resistant strains of Ps. aeruginosa were susceptible to both tobramycin and amikacin. Resistance to amikacin tended to be of relatively low magnitude (most had minimal inhibitory concentrations (MIC's) between 31 and 125 microng/ml), but organisms which were resistant to the administration of amikacin were usually resistant to the other two aminoglycosidic antibiotics as well.

Synergy between ticarcillin and tobramycin against Pseudomonas aeruginosa and Enterobacteriaceae in vitro and in vivo

The antibacterial activities of ticarcillin, carbenicillin, tobramycin, and gentamicin and of combinations of these antibiotics were measured against Pseudomonas aeruginosa and other gram-negative bacilli in vitro and in experimental mouse infections. Synergistic effects were produced by the penicillin/aminoglycoside combinations in growth inhibition tests and in bactericidal tests against many of the bacteria tested. Combinations of ticarcillin + tobramycin were more active in vitro than carbenicillin + gentamicin against P. aeruginosa but were no more active than the latter against other gram-negative bacilli. Ticarcillin + tobramycin and carbenicillin + gentamicin also demonstrated synergistic activities against P. aeruginosa, Escherichia coli, and Enterobacter cloacae in experimental mouse infection models. Thus, the penicillin/aminoglycoside combinations produced greater protective effects than the individual antibiotics against lethal intraperitoneal infections and also were more effective in reducing kidney counts of viable bacteria and kidney abscess formation in experimental pyelonephritis infections. As was the case in vitro, ticarcillin + tobramycin was more effective than carbenicillin + gentamicin against the experimental P. aeruginosa infections. The results of these in vitro and in vivo studies suggest that combined therapy with ticarcillin and tobramycin may be warranted in the treatment of serious infections due to P. aeruginosa and Enterobacteriaceae.

In vitro synergism between carbenicillin and aminoglycosidic aminocyclitols against Acinetobacter calcoaceticus var. anitratus

Acinetobacter caleoaceticus var. anitratus is a nonfermentative, gram-negative bacillus that has been demonstrated to cause severe infections, usually in hospitalized patients. Since mild to moderate resistance of A. calcoaceticus to one or more aminoglycosidic aminocyclitols has been noted to occur, a study was undertaken to evaluate the activity of combinations of carbenicillin with either kanamycin, tobramycin, or gentamicin against 28 isolates of A. calcoaceticus obtained from clinical sources. Synergism (defined as at least 100-fold-increased killing at 24 h by the combination as compared with the most efficacious of the individual antibiotics) was demonstrated against 26 of 28 strains of A. calcoaceticus with carbenicillin plus kanamycin and carbenicillin plus tobramycin and against 25 of 28 strains with carbenicillin plus gentamicin. The median increased killing for the 28 strains was 4.2 log(10) with carbenicillin plus kanamycin and with carbenicillin plus tobramycin and 3.1 log(10) with carbenicillin plus gentamicin. The most important determinant of synergistic potential of each combination was the level of resistance of each strain of A. calcoaceticus to the aminoglycoside component of the combination.

Aminoglycoside antibiotics and renal function: changes in urinary gamma-glutamyltransferase excretion

The urinary excretion of the renal proximal tubular enzyme, gamma-glutamyltransferase (gamma-GT), has been studied in 41 patients receiving tobramycin, gentamicin or streptomycin for a variety of infections. All patients receiving tobramycin or gentamicin have shown increased excretion of gamma-GT in the urine. Only 46% of those receiving streptomycin have shown an increase in gamma-GT excretion and this is of a lesser degree. A change in creatinine clearance which could only be explained by antibiotic administration was detected in three patients (2 on gentamicin, 1 on streptomycin). The degree of elevation of urinary gamma-GT activity was greater when the initial creatinine clearance was lower, and it is therefore suggested that those patients with pre-existing renal dysfunction should be monitored particularly carefully for signs of nephrotoxicity from these antibiotics. Urinary gamma-GT is a useful enzyme in the investigation of renal drug effects.

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.

["In vitro" activity of gentamicin, sisomicin and tobramycin against "pseudomonas aeruginosa" strains from hospital sources (author's transl)]

The in vitro activity of two amynoglicosidic antibiotics, tobramycin and sisomicin, against 215 strains of Pseudomonas aeruginosa isolated from clinical specimens during the years 1972-1975, was evaluated in comparison with that of gentamicin. Tobramycin showed a greater activity than sisomicin and gentamicin. At the concentration of 5 mu/ml tobramycin resulted inhibitory on 94% of strains and sisomicin and gentamicin on 78.6% and 49.8% of strains respectively. The susceptibility of Pseudomonas aeruginosa strains to gentamicin decreased from 1972 to 1975, with an increasing of resistant strains of 23%. In an evaluation of possible cross-resistance of 14 strains of Pseudomonas aeruginosa highly resistant (MIC greater than or equal to 25 gamma/ml) to gentamicin, 13 showed a high degree of resistance also to sisomicin; on the other hand 7 strains highly resistant to gentamicin was sensitive to tobramycin at concentration less than or equal to 10 gamma/ml.

Aminoglycoside-resistant enterococci

Thirty-four recent clinical isolates of Streptococcus faecalis were tested for sensitivity to amoxycillin, benzylpenicillin, streptomycin, kanamycin, gentamicin, tobramycin, and amikacin. Amoxycillin was two- to four-fold more active than benzylpenicillin and all strains were inhibited by low concentrations of the penicillins. The aminoglycosides were less active against the enterococci than were the penicillins and a significant number of strains were insensitive or relatively insensitive to one or more of the aminoglycosides. Thus, eight (23%) strains showed a high level of resistance to streptomycin and kanamycin (MIC greater 5000 microng/ml) but were sensitive to gentamicin, tobramycin, and amikacin. In addition, two strains of Strep. faecalis, isolated at different hospitals from patients who had received topical gentamicin therapy, were relatively resistant to gentamicin (MIC250 to 500 microng/ml) and were less sensitive also to the other aminoglycosides. Bactericidal synergy was demonstrated by amoxycillin/aminoglycoside combinations against the enterococci, provided that the test strain of Strep. faecalis was sensitive to the aminoglycoside in the combination. An exception to this was the combination of amoxycillin plus amikacin which was not synergistic against kanamycin-resistant strains of Strep. faecalis although these organisms were sensitive to amikacin in the growth inhibition tests. The gentamicin-resistant strains showed variable responses to amoxycillin/aminoglycoside combinations in tests for bactericidal synergy and were generally less sensitive than typical strains of Strep. faecalis.

Comparison of four aminoglycoside antibiotics in the therapy of experimental E. coli meningitis

Tobramycin, sisomicin, and amikacin were compared with gentamicin in the therapy of experimental E. coli meningitis in rabbits. Meningitis was produced in 40 animals by intracisternal injection of 10(5) E. coli. Three dosages of each antibiotic were administered intravenously over 8 hours. Serum and CSF samples were obtained at 0,2,4,6, and 8 hours for determination of aminoglycoside concentrations and CSF bacteria counts. The four aminoglycosides demonstrated comparable penetration into the CSF. The mean percent penetration (CSF conc./serum conc. X 100%) with the three dosages was 10 to 50% for gentamicin, 8 to 23% for tobramycin, 6 to 16% for sisomicin, and 11 to 23% for amikacin. This variation in penetration reflected individual differences in each dosage group and the increase in percent penetration that was observed during therapy. Sisomicin and gentamicin were consistently bactericidal in vivo. Mean CSF bacterial titers gentamicin were consistently bactericidal in vivo. Mean CSF bacterial titers decreased 3.07 logs in rabbits treated with sisomicin and 2.44 logs in animals treated with gentamicin. Even through CSF concentrations were comparable, the group treated with tobramycin demonstrated only a 0.64 log decrease and the amikacin group had a 0.45 log increase in mean CSF titers. The bactericidal effect of sisomicin appeared to be more rapid than that of gentamicin. During the first 2 hours of therapy CSF titers declined 1.02 logs in animals treated with sisomicin as compared to 0.37 log in animals receiving gentamicin even though gentamicin concentrations were higher (mean 7.4 vs. 4.1 gm./ml.). Sisomicin also demonstrated greater bactericidal activity than the other aminoglycosides in normal CSF in vitro. The results of this study suggest that sisomicin may be of value in the therapy of E. coli meningitis.

[Neuromuscular blocking properties of tobramycin, dibekacin and ribostamycin in man (author's transl)]

Neuromuscular blocking properties of tobramycin (Tobracin 60mg), dibekacin (Panmycin 100 mg) and ribostamycin (Vistamycin 1g) were studied in man during anesthesia and surgery by observing the effects of these antibiotics on muscle twitch tension. These antibiotics alone did not show any neuromuscular blocking action in the therapeutic doses. The neuromuscular blocking action of d-tubocurarine chloride was potentiated by the intravenous administration of 1g of ribostamycin. Tobramycin 60mg did not show remarkable effect, but dibekacin 100mg produced a slight potentiating effect on the action of d-tubocurarine. The enhancement of the action of d-tubocurarine was anatagonized by edrophonium and calcium. The action of succinylcholine chloride was not affected by tobramycin or ribostamycin, but it was potentiated transitorily by dibekacin. It is advisable to pay attention to possible respiratory depression when ribostamycin and dibekacin and used in combination with non-depolarizing muscle relaxants and/or anesthetics.

Chemically defined antimicrobial susceptibility test medium for Pseudomonas aeruginosa

A chemically defined growth medium containing physiological concentrations of magnesium and calcium ions was utilized in a microdilution procedure for antimicrobial drug susceptibility testing of Pseudomonas aeruginosa. Determinations of growth end points were simplified by use of sodium citrate as a sole carbon source and bromothymol blue as a pH indicator. Growth of the test organisms was detectable by a change in the indicator color from green to blue after alkalinization of the medium due to citrate utilization. Minimal inhibitory concentrations of amikacin, carbenicillin, gentamicin, and tobramycin were determined on 100 recent clinical isolates of Pseudomonas. Parallel determinations using the microdilution procedure and a conventional tube-broth dilution technique incorporating Mueller-Hinton broth with identical magnesium and calcium content generally agreed within one twofold dilution. Modal minimal inhibitory concentrations for susceptible strains using the microdilution method were: amikacin, 6 mug/ml; carbenicillin, 50 mug/ml; gentamicin, 1.5 mug/ml; tobramycin, 1.5 mug/ml. This modified microdilution technique allowed rapid, definitive minimal inhibitory concentration determinations, using growth end points defined by a color indicator change.

Resistance to gentamicin: a growing concern

Gentaminic was introduced in 1969 as a broad-spectrum aminoglycoside effective in vitro against a majority of aerobic gram-negative bacilli. In recent years gentamicin-resistant clinical isolates have become more prevalent. In our laboratory in 1975, 32% of Pseudomonas sp and 44% of indole-negative Proteus sp isolates were resistant to gentamicin. Resistance to tobramycin is also increasing; 24% of Escherichia coli and 28% of indole-negative Proteus sp isolates were found to be tobramycin-resistant. In addition, isolation of previously uncommon gentamicin-resistant species, ie, Proteus rettgeri and other indole-positive Proteus sp, from clinical specimens has increased dramatically in the past five years. This increase in gentamicin and tobramycin-resistant gram-negative bacilli serves as a constant stimulus for the development of new antimicrobial agents.

Antimicrobial resistance and R factors in Salmonella isolated from humans and animals in Georgia and South Carolina

Three hundred and five strains of Salmonella isolated from humans and 152 strains isolated from animals in Georgia and South Carolina during 1973 were examined for resistance to 14 antimicrobial agents. S typhimurium was the most frequently isolated serotype (30% of human isolates and 13% of animal isolates). The overall incidence of antimicrobial resistance was 16% in human starins and 21% in animal strains. The most commonly found resistance were to streptomycin (11% in human strains and 16% in animal strains) and tetracycline (10% in both human and animal isolates). A higher prevalence of resistance to streptomycin, spectinomycin, and nitrofurantoin, which are currently used primarily in veterinary medicine, was found in animal Salmonella. Conversely, resistance to ampicillin, cephalothin, and kanamycin, which are used mainly in human medicine, was found more frequently in human Salmonella. No resistance to nalidixic acid, gentamicin, tobramycin, rifampin, or trimethoprim was detected. Sixty-five percent of resistant strains from humans and 60% of resistant strains from animals transferred their resistance patterns partially or in toto.

Sensitivity of gram-negative bacteria to six aminoglycoside antibiotics

An in vitro study of the susceptibility of 201 newly isolated strains of gramnegative bacteria to six aminoglycoside antibiotics (kanamycin, amikacin, gentamicin, tobramycin, sisomicin and netilmicin) was performed by the twofold dilution method in fluid medium. Both the minimal inhibitory concentration and the minimal bacteridical concentration were determined. Overall, tobramycin seemed the most effective of the drugs studied. Netilmicin, the new derivative from sisomicin, compared favourably with the other drugs tested, but may, on theoretical grounds, offer the additional advantage of retained efficacy in the face of developing bacterial resistance. Not unexpectedly, amikacin appeared to be the most promising of the drugs studied in its action against Pseudomonas aeruginosa. Amikacin and netilmicin appeared to be the most effective of this group of antibiotics against Klebsiella species.

In vitro anti-mycobacterial activity of some new amino-glycoside antibiotics

Four new aminoglycoside antibiotics, namely gentamycin, amikacin, tobramycin and sisiomycin were tested against several pathogenic strains of so-called 'typical' and 'atypical' mycobacteria. Only amikacin exhibited considerable in vitro anti-mycobacterial activity against Mycobacterium tuberculosis and not against other mycobacterial pathogens. None of the other three antibiotics exhibited activity against any of the mycobacteria at low concentrations.

In vitro activity of netilmicin

The activity of netilmicin against a variety of bacteria was similar to that of gentamicin, sisomicin, and tobramycin, but it was less active than these three drugs against Pseudomonas aeruginosa. Synergy with penicillin G against enterococci was demonstrated.

An outbreak of infection caused by a gentamicin-resistant Staphylococcus aureus

An outbreak of infection caused by a strain of Staphylococcus aureus resistant to gentamicin and tobramycin and other antibiotics occurred in two wards in a hospital. Eight patients were colonized, of whom six had clinical infections. Previous administration of gentamicin appeared to predispose the patients to infection with the strain. Restriction of the use of gentamicin and tobramycin is essential to preserve their value in serious infections.

In vitro studies with netilmicin compared with amikacin, gentamicin, and tobramycin

Netilmicin, a new semisynthetic aminoglycoside derived by ethylation of the 1-N position of the deoxystreptamine ring of sisomicin, was tested in vitro with 4,070 strains of gram-negative bacilli isolated at the UCLA Medical Center during 1975 to 1976, using the agar dilution technique and an inoculum of approximately 10(4) organisms. Results were compared with those simultaneously obtained for amikacin, gentamicin, and tobramycin. Using Mueller-Hinton medium, inhibitory concentrations in broth correlated with those obtained by the agar dilution method except for Pseudomonas aeruginosa, where a 2- to 16-fold difference in susceptibility was noted. For most clinically significant Enterobacteriaceae and P. aeruginosa, the activity of netilmicin in vitro was comparable or superior to that of gentamicin, tobramycin, and amikacin with respect to potency by weight and achievable blood levels. Against gentamicin-resistant strains (MIC > 16 mug/ml), the activity of netilmicin paralleled that of amikacin with the exception of Providencia stuartii, which was inhibited by amikacin but not by netilmicin.

Antibacterial activity of ticarcillin, tobramycin and gentamicin alone and in combination against Pseudomonas aeruginosa in vitro

Using the biophotometer with ticarcillin no persistent bactericidal effect was found against Pseudomonas aeruginosa NCTC 10490. After addition of 1.2 microgram/ml gentamicin an increase of multiplication of bacteria was observed, but not after 1.2 microgram/ml tobramycin. With 6.2 microgram/ml tobramycin bactericidal effects lasted more than 24 h. In tube dilution test with Isotonic Sensi-test Broth out of 109 examined strains 51% were resistant to gentamicin, 16% to tobramycin and 4.5% to ticarcillin. If MIC values of gentamicin and tobramycin were calculated for magnesium-free media the resistance rate would be 10% for gentamicin and 3% for tobramycin. Combining subinhibitory doses of gentamicin or tobramycin with ticarcillin, most of the strains resistant to gentamicin and tobramycin became susceptible. The rate of inactivation of tobramycin by ticarcillin depends on the fluid into which they are placed. In combination therapy both antibiotics should be applied separately and immediately one after the other.

R plasmids coding for gentamicin, tobramycin, and carbenicillin resistance in Serratia, Klebsiella and Escherichia coli strains from a single clinical source

Serratia marcescens, Klebsiella sp. and Escherichia coli strains bearing transferable resistance to aminoglycoside antibiotics, namely, gentamicin, tobramycin, kanamycin, streptomycin, lividomycin, and carbenicillin, as well as to a series of more classical drugs, began to emerge in materials from the Frankfurt University Hospital. Plasmids from Serratia, Klebsiella, and E. coli exhibit a broad host range in that they are transferable to E. coli, Proteus mirabilis and Salmonella typhimurium receipients. They are not transferable to Pseudomonas aeruginosa, although in that species plasmids of gentamicin and tobramycin resistance, was well as of resistance to further drugs, were detected in that area as early as in 1973. High-level carbenicillinase has been identified in Serratia and Klebsiella plasmids associated with drug resistance to aminoglycoside antibiotics.

Effect of N-acetylcysteine on antibiotic activity and bacterial growth in vitro

The antibiotic bacerial inactivity of N-acetylcysteine (NAC) and its interaction with penicillin and aminocyclitol antibiotics was evaluated. NAC inhibited growth of both gram-negative and gram-positive bacteria. Strains of Pseudomonas aeruginosa were more susceptible than other microorgainsms tested. P. aeruginosa strains were inhibited synergistically by NAC and carbenicillin or ticarcillin. However, NAC antagonized the activity of gentamicin and tobramycin. These findings have implications for the combined clinical use of NAC and aerosolized antibiotics and are also important for the processing of sputum specimens in the microbiology laboratory.

In vitro synergy of cefamandole-tobramycin combinations

Twenty-five isolates of Staphylococcus aureus, 24 isolates of Escherichia coli, and 25 isolates of Klebsiella pneumoniae obtained from clinical material were tested in vitro for susceptibility to cefamandole, tobramycin and combinations of the two antibiotics utilizing an automated microdilution system. Synergistic or partially synergistic bactericidal effects of the combination were observed against 15 of the S. aureus isolates (60%), 23 of the E. coli isolates (96%), and 19 of the K. pneumoniae isolates (76%) tested. No antagonistic effects of the combination were noted. This study suggests that cefamandole-tobramycin combinations are capable of acting synergistically in vitro against certain gram-positive and gram-negative organisms and may have potential usefulness in clinical situations such as gram-negative rod and staphylococcal sepsis.

Antimicrobial activity in vitro of netilmicin and comparison with sisomicin, gentamicin, and tobramycin

The antimicrobial activity of netilmicin, a new semisynthetic aminoglycosidic aminocyclitol, was determined against 123 recent gram-negative clinical isolates susceptible to gentamicin and 60 isolates resistant to either sisomicin, gentamicin, or tobramycin. The minimal inhibitory concentrations and minimal bactericidal concentrations of netilmicin, sisomicin, gentamicin, and tobramycin against Pseudomonas, Escherichia coli, Klebsiella, Enterobacter, Proteus mirabilis, and indole-positive Proteus were, in general, quite similar. Gentamicin was the most active against Serratia. A total of 54, 67, and 88% of gentamicin-resistant Pseudomonas, Serratia, and Klebsiella, respectively, were susceptible to netilmicin. Strains of indole-positive Proteus, Acinetobacter, Providencia, and E. coli resistant to gentamicin were likely to be resistant also to netilmicin.

Comparative in vitro activity of nafcillin, oxacillin, and methicillin in combination with gentamicin and tobramycin against enterococci

The in vitro activity of nafcillin, oxacillin, and methicillin alone and in combination with gentamicin and tobramycin against 30 strains of enterococci was investigated. The penicillinase-resistant penicillins were less active than penicillin and ampicillin against the enterococci. Nafcillin was more active than oxacillin and methicillin. Sixty-six percent of strains were inhibited by nafcillin at 6.25 mug/ml, but none was inhibited by oxacillin and methicillin at the same concentration. At 12.5 mug/ml, 83, 16, and 0% were inhibited by nafcillin, oxacillin, and methicillin, respectively. By using a variety of criteria and analyses, it was shown that none of the antibiotic combinations studied demonstrated enhanced killing of all strains of enterococci. Nafcillin-gentamicin was the best combination, and enhanced killing was demonstrated against most strains. Oxacillin was more effective than methicillin when combined with gentamicin. Tobramycin was probably less effective than gentamicin in combination with the penicillinase-resistant penicillins against enterococci.

Alteration of effectiveness of antibiotics by anaerobiosis

In vitro antibiotic susceptibility tests of facultative organisms are routinely performed under aerobic conditions, despite the fact that many infections caused by these organisms occur in anaerobic areas, i.e., intra-abdominal absess. Experiments were performed aerobically and anaerobically to determine the susceptibility of E. coli, P. mirabilis, and K. pneumoniae to gentamicin, tobramycin, cephalothin, cefazolin, and cefamandole. Antibiotic sensitivities were determined by disc and agar dilution techniques in air and in anaerobic jars with CO2 absorbed. Tube dilution studies were performed in air and anaerobically and time-kill studies were done in aerobic and anaerobic broth. The amount of aminoglycoside required to inhibit bacterial growth was increased 4 to 20 times by anaerobiosis in 20 of 25 strains tested. Time-kill curves showed that bacterial killing by aminoglycosides was markedly impaired by anaerobiosis. Anaerobic conditions had no effect on the rate or extent of killing by cephalosporins. These data may have significance in determination of antibiotic susceptibility of facultative organisms under anaerobic tissue conditions. Antibiotic sensitivity testing done on these organisms in air may not reflect the actual state of antibiotic-bacterial interaction under conditions of the infection.

Tobramycin, amikacin, sissomicin, and gentamicin resistant Gram-negative rods

Sensitivities to gentamicin, sissomicin, tobramycin, and amikacin were compared in 196 gentamicin-resistant Gram-negative rods and in 212 similar organisms sensitive to gentamicin, mainly isolated from clinical specimens. Amikacin was the aminoglycoside most active against gentamicin-resistant organisms, Pseudomonas aeruginosa, klebsiella spp, Escherichia coli, Proteus spp, Providencia spp, and Citrobacter spp being particularly susceptible. Most of the gentamicin-resistant organisms were isolated from the urine of patients undergoing surgery. Gentamicin was the most active antibiotic against gentamicin-sensitive E coli, Proteus mirabilis, and Serratia spp. Pseudomonas aeruginosa and other Pseudomonas spp were most susceptible to tobramycin.

In vitro susceptibility of clinically important bacteria to amikacin: correlation of results of broth dilution and disk sensitivity tests and effect of medium composition

Minimal inhibitory concentrations of amikacin for 296 strains of gram-positive and gram-negative bacteria determined in Mueller-Hinton broth by the method recommended by the International Collaborative Study Group confirmed previous observations that amikacin is effective against current clinical isolates sensitive to gentamicin and that strains moderatly sensitive or resistant to gentamicin are often inhibited by clinically achievable concentrations. A simple linear regression analysis was used for comparison of zone sizes obtained with 10-mug disks of amikacin with minimal inhibitory concentrations obtained by the tube dilution technique. For 152 strains, including seven species of Enterobacteriaceae, the correleation coefficient (r) was -0.79 on Oxoid diagnostic senstivity test agar (which yields zone sizes similar to those obtained on Mueller-Hinton medium) and -0.78 on Oxoid isotonic sensitest agar (which contains cations in isotonic concentrations); the figures were -0.74 and -0.66, respectively, for 80 strains of Pseudomonas aeruginosa. Zone sizes for enteric gram-negative bacilli were on the average 2-3 mm smaller on isotonic sensitest agar than on diagnostic sensitivity test agar, and about 6 mm smaller for strains of P. aeruginosa and Providencia. Minimal inhibitory concentrations of amikacin can be estimated satisfactorily by extrapolation of results of agar diffusion tests, but the composition of the medium, particularly its calcium and magnesium content, should be defined precisely.

In vitro susceptibility of recently isolated gram-negative bacteria to gentamicin, sisomicin, tobramycin, and amikacin

The emergence of bacteria with R-factor-mediated resistance transferable to many strains of Pseudomononas aeruginosa, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, and some Proteus and Providencia species has been reported in hospitals in the Rhine-Main region of Germany. In a comparative study, 1,250 strains belonging to 12 clinically important bacterial species were tested for susceptibility to gentamicin, sisomicin, tobramycin, and amikacin by the tube dilution methods. Gentamicin, sisomicin, and tobramycin can still be employed effectively for the treatment of serious gram-negative infections. However, some findings related to Enterobacteriaceae and P. aeruginosa and demonstration of transfer by R-plasmids among P. aeruginosa, E. coli, K. pneumoniae, Enterobacter, and S. marcescens point to less favorable developments in the future. nearly all bacterial strains tests, including those resistant to other aminoglycosides, were susceptible in vitro to amikacin. This highly valuable antibiotic should be employed only when specifically indicated, since indiscriminate use would favor selective emergence of resistant bacteria in the future.

[New plasmid resistance of Staphylococcus aureus to aminosides (gentamicin, tobramycin, amikacin]

Recently, strains of Staphylococcus aureus resistant to gentamicin, tobramycin and amikacin have been discovered in several hospitals in France. These new resistances, of two different types, are of plasmid origin and of enzyme mechanism. This study describes their current incidence.

Biological activity of netilmicin, a broad-spectrum semisynthetic aminoglycoside antibiotic

Netilmicin (Sch 20569) is a new broad-spectrum semisynthetic aminoglycoside derived from sisomicin. Netilmicin was compared to gentamicin, tobramycin, and amikacin in a variety of in vitro test systems as well as in mouse protection tests. Netilmicin was found to be similar in activity to gentamicin against aminoglycoside-susceptible strains in both in vitro and in vivo tests. Netilmicin was also active against many aminoglycoside-resistant strains of gram-negative bacteria, particularly those known to possess adenylating enzymes (ANT 2') or those with a similar resistance pattern. Netilmicin was found to be markedly less toxic than gentamicin in chronic studies in cats, although gentamicin appeared less toxic in acute toxicity tests in mice. The concentrations of netilmicin and gentamicin in serum were compared in dogs after intramuscular dosing, and the pharmacokinetics including peak concentrations in serum were found to be similar.

Amikacin, an aminoglycoside with marked activity against antibiotic-resistant clinical isolates

A total of 319 clinical isolates known to be resistant to one or more aminoglycoside antibiotics were tested for their susceptibility to 10 aminoglycosides. The percentages of isolates found by an agar dilution method to be susceptible were: amikacin, 83.7%; tobramycin, 41.4%; butirosin A, 33.2%; dideoxykanamycin B, 32.6%; gentamicin C, 27.3%; lividomycin A, 17.6%; neomycin B, 10.7%; paromomycin, 10.3%; kanamycin A, 10.0%; and ribostamycin, 7.2%. The effectiveness of the antibiotics was related to their degree of resistance to bacterial enzymes; e.g., of the nine enzymes known to inactivate antibiotics containing 2-deoxystreptamine, amikacin was affected by one enzyme, tobramycin by five, and gentamicin and kanamycin by six. Examination of cell-free extracts from the 52 strains resistant to amikacin revealed that only four contained the amikacin-inactivating enzyme aminoglycoside-6'-acetyltransferase, a finding indicating that this mechanism of resistance is rare. Other experiments suggest that most amikacin-resistant strains, which are almost invariably resistant to all aminoglycosides, lack the ability to accumulate effectively either amikacin or presumably the other antibiotics intracellularly.

Susceptibility of current clinical isolates of Pseudomonas aeruginosa and enteric gram-negative bacilli to amikacin and other aminoglycoside antibiotics

The susceptibility of current clinical isolates of Pseudomonas aeruginosa and Enterobacteriaceae to amikacin and other aminoglycosides was tested by a standardized disk sensitivity method. Minimal inhibitory concentrations (MICs) were determined for all 200 isolates tested, and mean MICs were calculated for each of 10 bacterial species. Amikacin proved to be the most effective of six aminoglycosides against nine bacterial species; isolates of Proteus morganii were slightly more sensitive to gentamicin than to amikacin. Whereas 50% of the 200 isolates could be considered resistant to gentamicin (MIC, greater than 16 mug/ml), 94.4% of the 126 enteric gram-negative bacilli and all 74 isolates of P. aeruginosa were sensitive to amikacin. At a concentration of 8 mug/ml, gentamicin inhibited 50% and tobramycin inhibited 67% of the 200 isolates. At 16 mug/ml, amikacin inhibited 96.5% of the 200 isolates; the respective figures for kanamycin, aminosidine, and streptomycin were 28.5%, 26.5%, and 24%. The virtual absence of cross-resistance between amikacin and gentamicin and between amikacin and the other four aminoglycosides was confirmed.

In vitro activity of amikacin and ten other aminoglycoside antibiotics against gentamicin-resistant bacterial strains

Sixty-nine strains of gentamicin-resistant gram-negative bacilli obtained from different geographical sources were tested for susceptibility to 11 aminoglycoside antibiotics. From the results of determinations of minimal inhibitory concentrations, patterns of resistance were established for 45 strains of Enterobacteriaceae and 24 strains of Pseudomonas aeruginosa. Overall, 81% of the strains were sensitive to amikacin and 33% of the strains were sensitive to butirosin, the next most active compound. Results indicated that 54% of the P. aeruginosa strains were sensitive to amikacin and 33% were sensitive to tobramycin. From resistance patterns, enzymes responsible for inactivation of the antibiotics were deduced. The most common enzyme was aminoglycoside nucleotidyltransferase(2''), either alone or combined with either aminoglycoside phosphotransferase(3')-I or aminoglycoside phosphotransferase(3')-II. Aminoglycoside acetyltransferase(2) was identified exclusively in strains of Providencia stuartii. Specific enzymes could not be identified for 30 strains, 21 of which were P. aeruginosa.

Suceptibility of aminoglycoside-resistant gram-negative bacilli to amikacin: delineation of individual resistance patterns

Gram-negative bacilli isolated from clinical specimens submitted for culture in two Paris hospitals during 1974 were studied for susceptibility to six currently used aminoglycosides: kanamycin, neomycin, paromomycin, lividomycin, gentamicin, and tobramycin. Resistance patterns of strains of various species including those of Enterobacteriaceae, Pseudomonas, and Moraxella were determined, and the strains were grouped into eight resistance "phenotypes." In comparative studies of 807 strains belonging to different phenotypes, amikacin was markedly more active than any of the six other antibiotics; at concentrations of less than or equal to 4 mug/ml, it inhibited about 88% of the strains, including those resistant to gentamicin and tobramycin. Some amikacin-resistant strains were found among different species. The mechanism of resistance to amikacin of strains of Serratia and Moraxella group II was related to an N-acetylating enzyme. Amikacin can be expected to be useful as an alternative treatment of infections due to gram-negative bacilli sensitive to aminoglycosides and also, more particularly, for the treatment of patients infected with multiresistant strains.