In vitro study of netilmicin compared with other aminoglycosides

Netilmicin (Sch 20569) is an ethyl derivative of gentamicin C(1a) that is active against most Enterobacteriaceae, Pseudomonas aeruginosa, and Staphylococcus aureus isolates. Among 342 clinical isolates tested, all staphylococci; 92% of Escherichia coli, 93% of Klebsiella pneumoniae, and 92% of Enterobacter were inhibited by 0.8 mug or less of netilmicin per ml, but only 78% of P. aeruginosa were inhibited by 3.1 mug or less per ml. Most clinical isolates of enterococci, Serratia marcescens, and Providencia were not inhibited by 3.1 mug of netilmicin per ml. Like other aminoglycosides, the netilmicin in vitro activity was markedly influenced by the growth medium used, with activity decreased by sodium, calcium, and magnesium. Netilmicin was more active at alkaline pH. Addition of magnesium to Pseudomonas or Serratia pretreated with netilmicin produced inhibition of killing. Netilmicin was more active than gentamicin, sisomicin, tobramycin, or amikacin against E. coli and K. pneumoniae. Netilmicin inhibited growth of all gentamicin-resistant isolates of Klebsiella and Citrobacter tested, but only 73% of E. coli; Pseudomonas and Providencia were resistant to netilmicin. Most Serratia (95%) and indole-positive Proteus (83%) isolates were resistant to netilmicin but were inhibited by amikacin.

Activity of newer aminoglycosides and carbenicillin, alone and in combination, against gentamicin-resistant Pseudomonas aeruginosa

The in vitro activity of the aminoglycoside antibiotics tobramycin, sisomicin, amikacin, gentamicin, and netilmicin (SCH 20569) were compared against 26 gentamicin-resistant isolates of Pseudomonas aeruginosa cultured from hospitalized children. Tobramycin had the greatest activity on a weight basis, followed by sisomicin, gentamicin, amikacin, and netilmicin. All isolates were resistant to achievable concentrations of netilmicin and gentamicin, but 23% were inhibited by achievable concentrations of tobramycin, 8% by amikacin, and 4% by sisomicin. The combinations carbenicillin/tobramycin, carbenicillin/sisomicin, and carbenicillin/amikacin were synergistic for 92% of strains; antagonism was not encountered. These in vitro results suggest that tobramycin, sisomicin, or amikacin in combination with carbenicillin would be the safest initial regimen in the therapy of gentamicin-resistant Pseudomonas infections pending susceptibility studies.

Selection and characterization of strains of Staphylococcus aureus displaying unusual resistance to aminoglycosides

Susceptibility tests with aminoglycosides against Staphylococcus aureus have revealed discrepancies between the minimal inhibitory concentrations and the minimal bactericidal concentrations. To further evaluate these discrepancies, kill curves were performed against a susceptible strain of S. aureus with five different aminoglycosides (amikacin, kanamycin, tobramycin, gentamicin, sisomicin) at concentrations up to 16-fold above the minimal inhibitory concentration. Results revealed the presence of small subpopulations of cells capable of growth within 24 h in concentrations of aminoglycoside up to eightfold above the minimal inhibitory concentration for the parent strain. These subpopulations occurred at a frequency of >/=10(-7) parent cells, were not physiologically different from the susceptible parent strains, and were present in approximately one-half of 30 strains of S. aureus tested. The resistance of these subpopulations was approximately eightfold higher than that of the parent for all five aminoglycosides and was independent of concentration or type of aminoglycoside used to select them. This resistance was not due to extracellular degradation of drug and was stable over eight transfers in drug-free medium, except when selected by gentamicin or sisomicin.

[Sisomicin versus gentamicin. A comparison of antibacterial and pharmacokinetic properties (author's transl)]

In a comparison of the antibacterial in-vitro activity of sisomicin, gentamicin, and tobramycine, sisomicin showed a higher activity against E. coli, indole-positive Proteus spp. and organisms of the Klebsiella-Enterobacter-Serratia group, whereas tobramycine was superior against Pseudomonas spp. However, the differences in activity between sisomicin and gentamicin were only within one step of dilution which is hardly sufficient to guarantee a basic superiority of one or the other preparation for clinical purposes. In 12 healthy probands serum levels and renal elimination of sisomicin and gentamicin after a single intramuscular injection of 1 mg/kg body weight were investigated in a randomised change-over trial. With corresponding serum levels, almost identical elimination half-lives of 109 and 111 minutes, and recovery values of 84 and 88.4%, respectively, there were no significant differences in the pharmacokinetics of sisomicin and gentamicin. Thus in the antibacterial treatment of gentamicin-sensitive gram-negative organisms sisomicin presents an equally good alternative. With organisms only moderately sensitive or resistant to gentamicin, sisomicin, tobramycine and amikacin should be additionally tested.

Laboratory findings of tobramycin and their relation to clinical response

The susceptibilities of various clinical isolates to tobramycin were studied, and the applicability of these data to clinical situations was evaluated. The bactericidal nature of tobramycin was confirmed by killing curves, and the minimal inhibitory concentrations for 500 common pathogens were used to define its spectrum. Isolates inhibited by less than or equal to 5 mug/ml were considered to be sensitive to tobramycin. The clinical response of some patients was examined in relation to the minimal inhibitory concentration for the infecting organism and the serum and urine levels of tobramycin. The activity of tobramycin was compared with that of gentamicin against Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. Tobramycin was at least twice as active as gentamicin against P. aeruginosa. The effectiveness of tobramycin against S. aureus and E. coli was compared with that of other commonly used antibiotics, and its clinical role as an alternative to gentamicin in the "best guess" treatment of septicemia was considered. Lincomycin is often added to counter the ineffectiveness of tobramycin against streptococci and anaerobes such as Bacteroides species, but this combination was antagonistic against E. coli when tested in vitro by the checkerboard technique and its graphical display, the isobologram. Tobramycin was essentially similar to gentamicin in laboratory characteristics and clinical application but was more active against P. aeruginosa in general and against gentamicin-resistant strains in particular.

Bacteriological, clinical, and pharmacological investigations with tobramycin in patients with serious urinary tract infection

In studies of tobramycin an excellent effect was noted against many strains of bacteria of the family Enterobacteriaceae and of Pseudomonas aeruginosa and Staphylococcus derived from clinical sources. When the susceptibility of the strains to tobramycin was compared with susceptibility to other aminoglycoside antibiotics, tobramycin was clearly the most effective antibiotic of this group. Therapy of chronic urinary tract infection with tobramycin resulted in good clinical effects with no (or only slight) adverse reactions. When the parameters of recovery in patients treated with tobramycin were compared with those in other patients who had the same diseases but were treated with gentamicin, tobramycin yielded clinical results that were as good as (and occasionally better than) those produced by gentamicin. Studies of the pharmacokinetics of tobramycin showed a high rate of absorption from the muscles, a high rate of renal excretion, and effective therapeutic concentrations (higher than the minimal inhibitory concentration for the infecting strains) in renal tissue homogenates.

Tobramycin in treatment of infections due to Pseudomonas aeruginosa in patients with cystic fibrosis

The effectiveness and safety of tobramycin against infections due to Pseudomonas aeruginosa in patients with cystic fibrosis were studied in 15 patients with moderate-to-severe exacerbation of chronic pulmonary infection. Seventeen courses of treatment were given. Tobramycin (5-7.5 mg/kg per day) was administered intravenously in three divided doses per day (seven to 21 days). All specimens of sputum yielded moderate-to-heavy growth of P. aeruginosa; all isolates were inhibited by 4 mug of tobramycin/ml, and in seven cases isolates were not inhibited by 4 mug of gentamicin/ml. The mean levels of tobramycin in the blood were 4.4 mug/ml 0.5-1 hr and 0.8 mug/ml 7.5-8 hr after administration. All patients received an antistaphylococcal agent before, during, and after therapy. All patients except for two received 750 mg of carbenicillin/kg per day intravenously in six divided doses and 1 g of carbenicillin in a mask for inhalation three times a day. Both clinical and radiological improvement was noted in all patients. The white blood cell count, which was elevated in seven patients, returned to normal, and levels of blood gases improved. P. aeruginosa was eliminated from the sputum in five cases. No adverse side effects were noted.

Rapid microbiologic assay of tobramycin

Different, rapid, plate diffusion assays of tobramycin in serum were compared with each other and with an overnight assay. These assays were done with Klebsiella edwardsii var. atlantae, which was resistant to many antibiotics. One of the rapid assays of tobramycin tested used a 1:50-1:100 dilution of an overnight culture, and results were recorded after incubation for 5 hr at 35 C. Two shorter assays were also tested with a 1:10 dilution of an overnight assay. The 5-hr assay was as reproducible and as accurate as the overnight assay. The 2.5-hr assay at 35 C proved to be an inaccurate method. Incubation of the assay at 40 C instead of 35 C resulted in more rapid growth of K. edwardsii and in the production by tobramycin of larger and sharper zones of inhibition. The 2.5-hr assay at 40 C was as reproducible and as accurate as the 5-hr and the overnight assays.

Pharmacokinetics of tobramycin in rats with reduced renal parenchyma and in experimental shock

Ten days after four-fifths nephrectomy in rats (four-fifths of the renal parenchyma removed), the total and renal elimination constants of tobramycin were lowered by one-half. The biological half-life of tobramycin doubled. These results suggest that soon after nephrectomy the residual nephrons are able to increase the rate of excretion of tobramycin. The increase in mass of the remaining renal tissue between day 10 and day 80 after nephrectomy had no effect on the biological half-life, on the total and renal elimination constants, and on the renal clearance of tobramycin. During endotoxin shock and burn shock, concentrations of tobramycin in serum decreased at a slower rate. The highest serum concentrations of tobramycin were observed in rats with shock due to burns.

Pharmacokinetics and ototoxicity of gentamicin, tobramycin, and amikacin

The pharmacokinetics of gentamicin, tobramycin, and amikacin in inner ear fluids, serum, cerebrospinal fluid, and the compartments of the eye were studied and compared in guinea pigs. The concentrations of antibiotic were determined by microbiologic methods and were confirmed by the use of 14C-labeled gentamicin. Retention was clearly demonstrated in perilymph, in which the half-lives of gentamicin, tobramycin, and amikacin were 12, 11, and 10 hr, respectively. The concentrations of drug in perilymph were symmetrical and were many times higher than the concentrations of antibiotic in the brain. There was no difference between the concentration of drug in endolymph and that in perilymph. A linear relation between concentrations in the perilymph and the dosage of gentamicin was ascertained. Long-term treatment did not influence the pharmacokinetics of the three antibiotics in the inner ear. However, increased levels of drug in the inner ear in animals with uremia and in some animals with otitis media explained the increased ototoxicity that occurs in treatment of these two conditions. Suboccipital puncture and diuresis did not change the concentrations of aminoglycoside antibiotics in the inner ear. Antibiotics applied locally in the middle ear had high degrees of ototoxicity.

Pharmacokinetics of intravenously administered tobramycin in normal volunteers and in renal-impaired and hemodialyzed patients

The pharmacokinetics of intravenously administered tobramycin, an aminoglycoside antibiotic, were studied in 16 volunteers with normal renal function and 10 patients with different degrees of renal impairment. In all subjects, the disappearance of tobramycin from serum followed first-order kinetics, and the elimination rate constant decreased proportionally with increasing degree of renal impairment. The half-life average 82 min in normal subjects (endogenous creatinine clearance, greater than 80 ml/1.73 m2 per min) and was 33.7 hr in a totally anephric subject. Linear relations were defined between the half-life of tobramycin in serum and the reciprocal of the endogenous creatinine clearance and between the half-life of the drug and the concentration of serum creatinine. These relations may be used for dosage adjustment in patients with renal impairment. In hemodialyzed patients, the half-life decreased six- to ninefold during dialysis for 6 hr. The extent of tobramycin elimination by hemodialysis may be predicted from relations established between the elimination of tobramycin and that of creatinine and blood urea nitrogen. Dosage schedules must be compatible with the pharmacologic and pharmacokinetic properties of the antibiotic.

Tobramycin and ticarcillin therapy for exacerbations of pulmonary disease in patients with cystic fibrosis

Patients who had cystic fibrosis and acute infectious exacerbations of pulmonary disease produced by Pseudomonas aeruginosa were treated with a combination of tobramycin and ticarcillin. P. aeruginosa recovered from patients was inhibited by lower concentrations of both of these drugs than of gentamicin or carbenicillin. Thirteen courses of treatment were administered to 11 patients (mean, 14.4 days). A favorable response was seen in 11 of 12 completed courses of treatment. Improvement was associated with decreases in white blood cell count, temperature, and sedimentation rate. Adverse reactions were uncommon. Although P. aeruginosa was not eradicated from the sputum, the clinical results suggest that the combination of tobramycin and ticarcillin may be particularly useful for treatment of acute exacerbations of pulmonary disease in patients with cystic fibrosis from whom P. aeruginosa is isolated.

Aspects of the pharmacology and toxicology of tobramycin in animals and humans

The pharmacology and toxicology of tobramycin in animals and humans are reviewed. After intramuscular and intravenous administration, tobramycin diffuses throughout most body tissues and tissue fluids. Therapeutic concentrations can be obtained by intravitreal or intradural injections. Dogs tolerate intracisternal doses of 0.2 mg/kg without adverse reaction. The half-life of tobramycin in cochlear fluid of guinea pigs and in renal tissues of rats is significantly longer than the serum half-life in these species and is reflected in the ototoxic and nephrotoxic potential of tobramycin and other aminoglycosides. In man, the serum half-life of tobramycin is 2 hr; renal clearance, apparent volume of distribution, and recovery from urine are similar to those parameters for gentamicin. The serum half-life in neonates in prolonged (4.5-8.7 hr). Concentrations of tobramycin in serum are effectively reduced by hemodialysis, but peritoneal dialysis is less efficient in elimination of the antibiotic. Tobramycin crosses the placenta and is concentrated in the kidney and urine of the fetus.

Comparative in vitro activity of tobramycin, gentamicin, kanamycin, colistin, carbenicillin, and ticarcillin and clinical isolates of Pseudomonas aeruginosa: epidemiological and therapeutic implications

The susceptibility of clinical isolates of Pseudomonas aeruginosa to six antibiotics was related to the epidemiologic knowledge yielded by serotyping. The agar dilution method for determination of minimal inhibitory concentrations (MICs) and the Habs serotyping system were used. Effectiveness of each agent ranked highest to lowest as follows: tobramycin, colistin, ticarcillin, carbenicillin, gentamicin, and kanamycin. For many isolates, the MICs of gentamicin were close to the peak level in serum. Susceptibility to carbenicillin and to ticarcillin were highly correlated, with MICs of ticarcillin half those of carbenicillin. Similary, susceptibility to gentamicin and to tobramycin were correlated. For susceptible strains, MICs of gentamicin were two to four times greater than those of tobramycin. However, strains resistant to gentamicin were clearly in one of two groups: susceptible to tobramycin or resistant to the drug. Most strains resistant to both gentamicin and tobramycin were serotype 11, whereas those resistant gentamicin and kanamycin only were nonagglutinable.

Comparative in vitro activity of Sch 20656, netilmicin, gentamicin, and tobramycin

Sch 20656 and netilmicin (Sch 20569), two new semisynthetic aminoglycosides, were as active as gentamicin and tobramycin against Enterobacteriaceae. Against Pseudomonas aeruginosa, Sch 20656 was the least active, whereas netilmicin was active against many highly gentamicin-resistant isolates.

Susceptibility of nonfermentative gram-negative bacilli to tobramycin

There has been increasing interest in the pathogenic role of nonfermentative gram-negative bacilli in human infections. Except for Pseudomonas aeruginosa, the susceptibility pattern of these organisms to tobramycin has not been evaluated thoroughly. The activity of tobramycin, as compared with that of gentamicin, was tested by the serial broth dilution technique against 178 isolates of nonfermentative gram-negative bacilli obtained from various sources. P. aeruginosa, Pseudomonas stutzeri, Acinetobacter calcoaceticus var. anitratum (Herellea vaginicola), A. calcoaceticus var. Iwoffi (Mima olymorpha), Pseudomonas alcaligenes, and Pseudomonas acidovorans accounted for 82% of all cultures tested. The vast majority of these organisms were susceptible to both tobramycin and gentamicin. Resistance was most common with Alcaligenes odorans; six of 12 isolates were resistant to gentamicin and tobramycin. There was only one isolate of Pseudomonas diminuta; it was highly resistant to both antibiotics.

Laboratory and clinical evaluation of tobramycin, a new aminoglycoside antibiotic

The in vitro susceptibility to tobramycin of 1,466 bacterial isolates, including several genera and strains of common pathogenic gram-negative bacilli and Staphylococcus aureus, was determined by conventional serial dilution methods. Tobramycin exhibited marked in vitro antibacterial activity; almost all of the isolates tested were inhibited by less than or equal to 2.5 mug of the drug/ml. The antibiotic was used in the treatment of several types of infection in 29 patients with normal renal function. Observations were made on the therapeutic and secondary effects of the drug and on possible toxicity to the eighth cranial nerve. Clinical therapeutic results were satisfactory, The hearing of one patient was impaired, but the relation of this effect to tobramycin therapy is uncertain. Serum levels of the antibiotic were measured in 10 patients to investigate whether the drug accumulated during repeated administration. Mean serum levels of tobramycin 1 hr after intramuscular injection of 1.0 mg/kg were between 2.5 and 3.0 mug/ml on days 1, 3, and 6 of therapy. No accumulation of the antibiotic was observed after repeated administration.

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.

Penetration of tobramycin sulfate in the aqueous humor of the rabbit

The intraocular penetration of tobramycin sulfate, a new aminoglycoside antibiotic, was evaluated in rabbits following subconjunctival injection. The mean tobramycin sulfate concentration in the aqueous humor 60 minutes after a single 5-mg dose was 5.5 mug/ml, as compared to a mean concentration of 6.7 mug/ml following a single 10-mg dose. These levels exceed the minimal inhibitory concentration (MIC) for most Pseudomonas species and a variety of other Gram-negative bacilli recovered from clinical infection. No anterior segment changes that may be attributed to the antibiotic could be demonstrated in the injected eye.

Gentamicin and tobramycin--an in vitro comparison using 1400 clinical isolates

The in vitro antimicrobial activities of tobramycin and gentamicin against 1400 bacterial isolates from clinical material were compared. The minimum inhibitory concentrations were determined by an agar dilution technique. Both of these aminoglycoside antibiotics had a similar spectrum of activity, being highly active against most species of aerobic Gram-negative bacilli and Staphylococcus aureus. Gentamicin was more active than tobramycin against most species of enterobacteria but tobramycin was more active against Pseudomonas aeruginosa and a proportion of Klebsiella isolates. For most isolates, the differences in activity between gentamicin and tobramycin were small.

Multiresistant plasmids from Pseudomonas aeruginosa highly resistant to either or both gentamicin and carbenicillin

High-level resistance to gentamicin and carbenicillin was found in 30 and 10.7%, respectively, of Pseudomonas aeruginosa strains, especially in isolates from urine. In 23 out of 25 strains tested, these resistances were R mediated and linked to multiresistant plasmids, carrying genes for resistances to five other aminoglycosides, tobramycin, kanamycin, neomycin, streptomycin, and spectinomycin, and for resistances to chloramphenicol, tetracycline, sulfonamides, and mercury chloride. Carbenicillin resistance was unstable in Pseudomonas, and in its presence the multiresistant plasmids had a host range extended to the Enterobacteriaceae (group I plasmids). Otherwise they were transferable intragenerically only (group II plasmids). The extended host range plasmids were, as a rule, in fi(-) incompatibility class A-C. Segregants incompatible with both class A-C and P plasmids were detected. The beta-lactamase specified by the carbenicillin marker was of the TEM-like type. Multiple linkages of resistance determinants to the aminoglycosides were concomitantly present in most of the plasmids. Results from the bioassay indicated the presence of at least two aminoglycoside-inactivating enzymes.

[Tobramycin sensitivity of bacteria from the Klebsiella-Enterobacter group (author's transl)]

In 300 strains of the Klebsiella-Enterobacter group which were isolated from pathological material from the Bonn University Hospitals in early 1975, the sensitivity to tobramycin was investigated comparatively in a serial dilution test and agar diffusion test. 80% of the strains were seen to be classifiable as sensitive and further 3% as moderately sensitive. The rate to resistance corresponds to that of gentamycin. No difference in resistance behaviour was ascertained in the two bacterial species investigated. However, strains isolated from the urogenital tract are significantly more resistant than pathogens isolated from the respiratory tract.

Tobramycin: in vitro and clinical evaluation in 30 patients

Clinical evaluation of intramuscular tobramycin was accomplished in 30 patients with respiratory, soft tissue, urinary tract, bone or septicemic infections due to gram negative bacilli. Median sensitivity to tobramycin of Pseudomonas aeruginosa isolates (19 strains) was 0.62 mug/ml and range 0.31-2.5 mug/ml; less activity was observed for Escherichia coli, Proteus mirabilis, Klebsiella pneumoniae and Enterobacter species isolates but median minimum inhibitory concentrations were less than or equal to 2.5 mug/ml. Therapy resulted in clinical and bacteriologic cures in 16 patients (53 per cent) including 13 of 16 (181 per cent) with urinary tract infections; 9 of the 14 patients who did not obtain bacteriologic cure had satisfactory clinical responses. Tobramycin was effective for selected gram negative bacillary infections and particularly for P. aeruginosa.

Colonization of the intestinal tract of conventional mice with Candida albicans and treatment with antifungal agents

Conventional mice inoculated with Candida albicans per os were unable to maintain this organism in the intestinal tract as judged by decreasing numbers of yeast recoverable from feces. After inoculation with 10(7) cells/mouse, fecal counts ranged from 10(5) cells per g of feces to 5 x 10(3) cells per g of feces during a 12-day experimental period. Addition of various antibiotics to the drinking water did not result in any improvement in maintenance or stability of the gut population. A combination of X irradiation and administration of tobramycin or gentamicin, however, resulted in a stable population of C. albicans in the intestinal tract, with cell counts in the feces remaining constant at a level of about 10(6)/g of feces for a period of 10 to 15 days. The usefulness of this model in assessing the effect of experimental drugs on C. albicans infections of the gut was demonstrated by the fact that treatment with a new antifungal antibiotic (A9145), amphotericin B, 5-fluorocytosine, or nystatin resulted in a reduction in the fecal counts of C. albicans from experimentally infected animals.

Effect of tobramycin on urinary gamma-glutamyltransferase activity: Studies in a case of renal carcinoma

Gamma-Glutamyltransferase activity was studied in a man presenting with recurrent septicemia owing to pyonephrosis and renal carcinoma. Increased activity in the urine was ascribable to administration of the aminoglycoside antibiotic, tobramycin. That the renal carcinoma did not contribute to the increased values was confirmed by homogenization and enzyme histochemistry of the tumor. Although the activity of this enzyme in serum was greater than normal, this persisted postoperatively, and thus was not related to the renal carcinoma.

Comparative evaluation of five aminoglycosides for treatment

Gentamicin, tobramycin, sisomicin, amikacin and kanamycin are compared for therapeutic use by correlating the in vitro minimal inhibitory concentration with the in vivo concentration of the antibiotic attainable in the serum at half the time intervall between two administrations of the drugs. Using this method of evaluation, E. coli, Enterobacter and sensitive strains of the Klebsiella and Pseudomonas species can be treated in principle equally well with gentamicin, tobramycin and sisomicin, and to a large degree also with amikacin. Gentamicin, sisomicin and amikacin are useful against Serratia infections; but in each case, a higher dosage is needed. Within the family of aminoglycosides, sisomicin has a noteworthy activity against Enterobacter and Proteus species, while tobramycin is most outstanding against pseudomonas strains. Amikacin is especially useful against infections involving Klebsiella, Providencia and Pseudomonas strains with resistance to nearly all antibiotics including the other aminoglycosides.

[ANT(4')I: a new aminoglycoside nucleotidyltransferase found in "staphylococcus aureus" (author's transl)]

A new nucleotidyltransferase has been obtained from clinical isolates of kanamycin, amikacin, tobramycin, neomycin and lividomycin resistant strains of Staphylococcus aureus. The enzyme has been purified by affinity chromatography then characterized in a kinetic and physicochemical point of view. The results obtained support that the target of this enzyme is the 4'-hydroxyl group of a number of aminoglycosides. We propose to call it ANT(4')I.

Proteus species isolated from human eyes

Of 34 species of Proteus isolated from human eyes, 29 (85%) were P. mirabilis and five (15%) were P. morganii. In vitro antibiotic sensitivity studies showed that gentamicin best controlled both P. mirabilis and P. morganii of all the antibiotics tested. In vivo tests on experimental Proteus infections of rabbit coreas, treated with gentamicin and tobramycin, yielded comparable clinical results, but gentamicin was more effective in eliminating the organism from the experimental lesions.

Gentamicin-resistant Pseudomonas aeruginosa and Serratia marcescens in a general hospital

From Aug. 1, 1974, to July 31, 1975, inclusive, 19-1% of Pseudomonas aeruginosa and 50-0% of Serratia marcescens isolates from a general hospital were gentamicin resistant as determined by standardised disc testing; 80% of 118 different clinical isolates showed minimum inhibitory concentrations larger than or equal to 16 mug/ml of gentamicin in agar-dilution testing. All gentamicin-resistant S. marcescens and 79% of Ps. aeruginosa isolates were susceptible to amikacin, while tobramycin and sisomicin were shown to have little advantage over gentamicin. A wide variety of immunotypes was found for Ps. aeruginosa, but only two predominant types were found for S. marcescens. Most isolates originated from the urinary tract, but nine which came from urine or wounds were also isolated from blood. Resistant strains seemed to be as virulent as sensitive strains. A common source was not found. Clinical improvement was noted in 13 of 17 patients treated with amikacin. The high frequency of gentamicin resistance is a continuing problem in this hospital and soon may be in others.

Outer hair cell loss and alterations in glycogen due to tobramycin sulfate

A total of 26 albino guinea pigs were treated with 200 mg/kg/day of tobramycin sulfate. Animals were killed at various intervals of up to three weeks after seven days treatment. Outer hair cell (OHC) loss of the organ of Corti was evaluated by surface preparation techniques, and glycogen was assessed with PATCO- and PAS-stained sections. We have concluded that permanent damage of OHCs is most prevalent in areas that normally have the least amount and the smallest granule size of glycogen. With treatment, these susceptible areas are the least responsive in terms of an early increase in glycogen production. Furthermore, by 21 days after seven days of treatment, these same areas will reveal an almost total loss of glycogen at a time when damage is maximal.

[Bacteriological studies in outpatients with acute urinary tract infections with particular reference to the resistance spectrum (author's transl)]

Bacteriological investigation of urinary samples from 1,926 non-hospitalised patients with documented or suspected acute urinary tract infection revealed organisms pathogenic for the urinary tract in 56.4% of the patients, who came from various parts of West Germany. Prevalent pathogens were E. coli (69%) and Proteus mirabilis (14%). E. coli and P. mirabilis demonstrated a low rate of resistance against ampicillin, the cephalosporines, gentamicin, tobramycin, and also against nitrofurantoin, nalidixine acid and trimethoprimsulfamethoxazole. The situation was more unfavourable in the case of Klebsiella and indolpositive Proteus species however, there being a noticeably high proportion of strains resistant to gentamicin and tobramycin.

In vitro activity, synergism, and testing parameters of amikacin, with comparisons to other aminoglycoside antibiotics

The activity of the new aminoglycoside antibiotic, amikacin, was evaluated in vitro against 219 clinical bacterial isolates. One hundred eighty-nine of the 219 strains had agar dilution minimal inhibitory concentration values of 8.0 mug/ml or less for amikacin. Comparative agar dilution studies were performed for gentamicin, kanamycin, and tobramycin. Gentamicin was the most active overall, but tobramycin and amikacin also had significant activity against most bacterial groups. The effects of divalent cations on the susceptibility of Pseudomonas aeruginosa to amikacin were evaluated, and the minimal inhibitory concentration values varied sixfold over a range of divalent cation concentrations from 0.2 to 8.75 mg%. The effects of media and inoculum size on disk susceptibility testing with amikacin were also evaluated. In addition, a synergistic interaction between carbenicillin and amikacin against P. aeruginosa was demonstrated. Amikacin appears to be a promising new broad spectrum antimicrobial agent.

[Antimicrobial effectiveness of sisomicin. I: In vitro activity of sisomicin compared with gentamicin, tobramycin, amikacin and kanamycin (author's transl)]

The aminoglycosides sisomicin, gentamicin, tobramycin, amikacin and kanamycin are highly active against staphylococci including the penicillinase-positive strains. Sisomicin is more effective than amikacin and kanamycin. Mixed infections with staphylococci and Enterobacteriaceae or Pseudomonas aeruginosa are thus on indication for treatment with sisomicin or other aminoglycosides. Infections with E. coli, Enterobacter, susceptible Klebsiella, and susceptible Pseudomonas strains can be treated with sisomicin, gentamicin or tobramycin. In such cases sisomicin is the most effective antibiotic because of its high antimicrobial activity. In infections with these organisms amikacin can also be used for treatment especially if there is resistance to other aminoglycosides. In hospital-acquired infections with Serratia marcescens amikacin and sisomicin are the drugs of choice. Both aminoglycosides have to be given in high doses in infections with Serratia because of the high inhibitory concentration for Serratia. Sisomicin demonstrates a high antimicrobial activity particularly against indole-positive Proteus species such as Proteus vulgaris and Proteus morganii, Enterobacter, and gentamicin-sensitive Pseudomonas strains. In infections with Pseudomonas aeruginosa tobramycin is the most effective bactericidal antibiotic. Amikacin is the drug of choice against gentamicin-resistant Pseudomonas strains which are also not infrequently resistant to other aminoglycosides. The low proportion of resistance to sisomicin of 7,6% in 370 organisms is only exceeded by amikacin with a rate of 0,6% (resistance to tobramycin 11,4%, gentamicin, 13,2% and kanamycin 42,4%). The low rate of resistance and the high antimicrobial activity are essential advantages of sisomicin.

Commentary: An appraisal of tobramycin usage in pediatrics

Tobramycin is a newly marketed aminoglycoside which closely resembles gentamicin in antimicrobial activity, pharmacology, clinical efficacy, and toxicity. It is somewhat more active in vitro against Pseudomonas aeruginosa than is gentamicin and may have a lower ototoxic potential. Tobramycin should be considered a limited-purpose drug for pediatric patients until greater clinical experience has been gained. At the present time the major indication for its use is for treatment of infections caused by coliforms or pseudomonas resistant to kanamycin and gentamicin. Demonstration of in vitro susceptibility is mandatory because resistance to tobramycin and the other aminoglycosides may be mediated by the same episome (R-factor). The recommended dosage is 2 mg/kg every 12 hours (4 mg/kg/day) intramuscularly or as a two-hour intravenous infusion to neonates, with the possible exception of full-term infants over seven days of age who may require administration every eight hours. Beyond the neonatal period, the dosage should be 1.0 to 1.5 mg/kg every eight hours (3 to 4.5 mg/kg/day). Larger dosages may be required for treatment of meningitis, but presently there is no information on which to base a recommendation. Neither is there experience with intrathecal use in infants. It is desirable to monitor tobramycin serum concentration to be certain that peak values are within the therapeutic range of 3 to 8 mug/ml. Dosage must be reduced in patients with impaired renal function and monitoring of serum concentrations is imperative. All patients should be evaluated for evidence of renal and eighth nerve toxicity.

Gentamicin resistance in Staphylococcus aureus

A clinical isolate of Staphylococcus aureus was resistant to gentamicin, kanamycin, and tobramycin by virtue of the production of a drug-inactivating enzyme. Attempts to establish the cellular location of the genetic determinants for enzyme synthesis have given equivocal results.

["In vitro" determination of bacterial sensitivity to tobramycin (author's transl)]

The authors report the tobramycin minimal inhibitory concentration (MIC) of 264 strains (dilution in Mueller-Hinton agar). Staphylococcus aureus (96 strains) have a MIC of 0.02 mcg/ml, 80 % being inhibited by 0.07 mcg/ml. Most Enterobacteriaceae (70 strains) are sensitive to 0.7 mcg/ml ; Providencia are the most resistant (MIC 11.88 mcg/ml). Among Pseudomonas aeruginosa (98 strains, MIC : 11.57 mcg/ml) 50 % are inhibited by 0.30 mcg/ml, 80 % by 18.8 mcg/ml. Relationships of MIC to zone diameters with the use of 10 mcg (D 10) and 30 mcg (D 30) tobramycin discs allow to draw the regression line which is respectively : y = -- 0,331 x + 6,64 and y = -- 0.263 x + 7,68. The strains are sensitive with an inhibition diameter greater than or equal to 16 mm (D 10) or greater than or equal to 18.5 mm (D 30).

Tobramycin: a review of its antibacterial and pharmacokinetic properties and therapeutic use

SYNOPSIS

Tobramycin is a new aminoglycoside antibiotic with a broad antibacterial spectrum in vitro, and pharmacokinetic properties similar to those for gentamicin. Tobramycin is more active than gentamicin against Pseudomonas aeruginosa and active against many gentamicin resistant strains, but is not active against enterobacteriaceae resistant to gentamicin. Theoretically, tobramycin has an advantage over gentamicin against infections caused by P. aeruginosa, but any advantage in clinical practice has yet to be adequately demonstrated. Clinical experience with tobramycin is considerably less than with gentamicin. Whilst tobramycin appears to offer no clear advantages over gentamicin against sensitive organisms it is indicated in infection caused by strains of P. aeruginosa which are resistant to gentamicin, but sensitive to tobramycin. Like gentamicin, tobramycin acts synergistically with corbenicillin and the cephalosporins. The efficacy of the tobramycin-carbenicillin combination has been shown in endocarditis caused by P. aeruginosa which was unresponsive to gentamicin plus carbenicillin. Ototoxicity and nephrotoxicity similar to that seen with other animoglycosides have been encountered in therapeutic trials with tobramycin and wider clinical experience is necessary to determine the relative incidence of these side-effects with gentamicin and tobramycin used under similar conditions. Antimicrobial activity: In comparative studies, in vitro, tobramycin is more active than gentamicin against clinical isolates of Pseudomonas aeruginosa. Similarly, the inhibitory index, which is the ratio between the serum concentration attained at usual therapuetic doses and the minimum inhibitory concentration, for Pseudomonas aeruginosa is higher for tobramycin than for gentamicin. Against Gram-negative bacteria other than Pseudomonas spp. the spectrum of activity of tobramycin is similar to that of gentamicin. For most species the activity of tobramycin is slightly less than that of gentamicin. Gentamicin is consistently more active than tobramycin against Serratia marcescens. Like other aminoglycoside antibiotics, tobramycin is active in vitro in low concentrations against Staphylococcus aureus. Tobramycin is essentially inactive against Streptococcus pyogenes, Streptococcus faecalis and Streptococcus pneumoniae (pneumococci). Maner aminoglycosides and of other antibiotics against various bacteria in vitro, but comparisons between studies cannot always be interpreted literally because the activity of many antibiotics in vitro, including tobramycin, is influenced by the nature of the culture media and the presence of certain salts. The sensitivity of P. aeruginosa to tobramycin is influenced by the magnesium, and calcium content of the culture media whilst that of all species is reduced by sodium ions. Wide variations in the concentration of these ions may result in divergent MIC values and an inappropriate choice of antibacterial agent to treat pseudomonas infection...

Characterization of a nosocomially significant, multiple drug-resistant strain of Serratia marcescens

A multiple drug-resistant strain of Serratia marcescens (bacteriocin type 18) was isolated from three clinical patients. The isolates were found to carry a conjugally nontransferable, nonmobilizeable resistance plasmid (R-plasmid) with resistance-(r)-determinants against ten antimicrobial drugs: ampicillin, carbenicillin, chloramphenicol, gentamicin, kanamycin, neomycin, streptomycin, tobramycin, triple sulfonamides, cotrimoxazole, and--possibly--nalidixic acid, as determined with exposure to 'curing' agents (ethidium bromide, acridine orange, and sodium dodecyl sulfate) and by the high rate of spontaneous loss of r-determinants. Dyebuoyant density centrifugation allowed recovery of R-plasmid DNA that measured roughly 24 mum in contour length; after 'curing' with concomitant loss of 9 r-determinants, the contour length of the R-plasmid DNA of one isolate (No. SE 154) had decreased to roughly 15 mum, and none was detected in the sole variant of the isolate that spontaneously had lost 11 r-determinants.