Colonization with gentamicin-resistant Pseudomonas aeruginosa, pyocine type 5, in a burn unit

Multidrug- and Carbapenem-Resistant Pseudomonas aeruginosa in Children, United States, 1999-2012

BACKGROUND

Pseudomonas aeruginosa is a common cause of healthcare-associated infection. Multidrug-resistant (MDR) (>3 classes) and carbapenem-resistant (CR) P aeruginosa are significant threats globally. We used a large reference-laboratory database to study the epidemiology of P aeruginosa in children in the United States.

METHODS

Antimicrobial susceptibility data from the Surveillance Network were used to phenotypically identify MDR and CR P aeruginosa isolates in children aged 1 to 17 years between January 1999 and July 2012. Logistic regression analysis was used to calculate trends in the prevalence of MDR and CR P aeruginosa. Isolates from infants (<1 year old) and patients with cystic fibrosis were excluded.

RESULTS

Among the isolates tested, the crude proportion of MDR P aeruginosa increased from 15.4% in 1999 to 26% in 2012, and the proportion of CR P aeruginosa increased from 9.4% in 1999 to 20% in 2012. The proportion of both MDR and CR P aeruginosa increased each year by 4% (odds ratio [OR], 1.04 [95% confidence interval (CI), 1.03-1.04] and 1.04 [95% CI, 1.04-1.05], respectively). In multivariable analysis, both MDR and CR P aeruginosa were more common in the intensive care setting, among children aged 13 to 17 years, in respiratory specimens, and in the West North Central region. In addition, resistance to other antibiotic classes (aminoglycosides, fluoroquinolones, cephalosporins, and piperacillin-tazobactam) often used to treat P aeruginosa increased.

CONCLUSIONS

Rates of MDR and CR P aeruginosa infection in children are rising nationally. Aggressive prevention strategies, including instituting antimicrobial stewardship programs in pediatric settings, are essential for combating antimicrobial resistance.

Nosocomial Infection Caused by Antibiotic-Resistant Organisms in the Intensive-Care Unit

Resistance to antimicrobial agents is an evolving process, driven by the selective pressure of heavy antibiotic use in individuals living in close proximity to others. The intensive care unit (ICU), crowded with debilitated patients who are receiving broad-spectrum antibiotics and being cared for by busy physicians, nurses, and technicians, serves as an ideal environment for the emergence of antibiotic resistance. Problem pathogens presently include multiply resistant gram-negative bacilli, methicillin-resistantStaphylococcus aureus, and the recently emerged vancomycin-resistant enterococci. The prevention of antimicrobial resistance in ICUs should focus on recognition via routine unit-based sur veillance, improved compliance with handwashing and barrier precautions, and antibiotic-use policies tailored to individual units within hospitals.

Infection control in the burn unit

The survival rates for burn patients have improved substantially in the past few decades due to advances in modern medical care in specialized burn centers. Burn wound infections are one of the most important and potentially serious complications that occur in the acute period following injury. In addition to the nature and extent of the thermal injury influencing infections, the type and quantity of microorganisms that colonize the burn wound appear to influence the future risk of invasive wound infection. The focus of medical care needs to be to prevent infection. The value of infection prevention has been acknowledged in organized burn care since its establishment and is of crucial importance. This review focuses on modern aspects of the epidemiology, diagnosis, management, and prevention of burn wound infections and sepsis.

Properties of R plasmids determining gentamicin resistance by acetylation in Pseudomonas aeruginosa

Two clinical isolates of Pseudomonas aeruginosa, one a pyocin type 5 strain from Atlanta, could transfer gentamicin resistance by conjugation. Donor and recipient strains inactivated gentamicin by acetylation. The R plasmids, pMG1 and pMG2, also determined resistance to sisomicin, another substrate of gentamicin acetyltransferase I, sulfonamides, and streptomycin, but not resistance to kanamycin, neomycin, tobramycin, butirosin, or BB-K 8. They were transmissible to many strains of P. aeruginosa, including a Rec(-) strain, but not to Escherichia coli or other enterobacteriaceae. These R plasmids were compatible with R plasmids transmissible to P. aeruginosa from E. coli, including members of C, N, P, and W incompatibility groups. From a strain carrying pMG1 and a compatible plasmid, pMG1 was transferred independently but transfer of the second plasmid often resulted in cotransfer of pMG1. In contrast, pMG1 and pMG2 were incompatible with pseudomonas R plasmids R931 and R3108, and with R931 they readily formed recombinant plasmids. The four plasmids in this incompatibility group determine additional biological properties, including resistance to inorganic and organic mercury compounds, to ultraviolet light, and to certain deoxyribonucleic acid phages. pMG1 and pMG2 also phenotypically inhibited pyocin production. Consequently such R plasmids alter the phage and pyocin types of their host strains.

The epidemiology of burn wound infections: then and now

Burn wound infections are a serious complication of thermal injury. Although pneumonia is now the most important infection in patients with burns, burn wound infection remains a serious complication unique to the burn recipient. The methods for managing thermal injury have evolved during the past 50 years. This evolution has been accompanied by changes in the etiology, epidemiology, and approach to prevention of burn wound infections. In the 1950s, 1960s, and 1970s and into the mid-1980s, burn wounds were treated by the exposure method, with application of topical antimicrobials to the burn wound surface and gradual debridement with immersion hydrotherapy. As early burn wound excision and wound closure became the focal point of burn wound management, accompanied by a change from immersion hydrotherapy to showering hydrotherapy, the rate of burn wound infection appeared to decrease. Few epidemiologic studies have been done since this change in the approach to management of thermal injury. There are few data on the epidemiology of burn wound infections from the era of early excision and closure. Data are needed on infection rates for excised and closed burn wounds, the etiologies of these infections, and the epidemiology and the prevention of such infections. Additional studies are needed on the indications for topical and antimicrobial prophylaxis and selective decontamination of the digestive tract.

Human diseases caused by foodborne pathogens of animal origin

Many lines of evidence link antimicrobial-resistant human infections to foodborne pathogens of animal origin. Types of evidence reviewed include: (1) direct epidemiologic studies; (2) temporal evidence; (3) additional circumstantial evidence; (4) trends in antimicrobial resistance among Salmonella isolates; and (5) trends in antimicrobial resistance among other pathogens, such as Campylobacter jejuni. Commensal microorganisms in animals and humans may contribute to antimicrobial resistance among pathogens that cause disease among humans. For instance, enterococci of food-animal origin, particularly strains that are vancomycin resistant, have been linked to strains found in the human gastrointestinal tract. The latent period between the introduction of a given antimicrobial and emergence of resistance varies considerably, but once the prevalence in a population reaches a certain level, control becomes extremely difficult.

An outbreak of methicillin resistant Staphylococcus aureus on a burn unit: potential role of contaminated hydrotherapy equipment

OBJECTIVE

To report a multi-institution outbreak caused by a single strain of methicillin-resistant Staphylococcus aureus (MRSA).

OUTBREAK

Between September 19 and November 20, 1996 an index case and five secondary cases of nosocomial MRSA occurred on a 26 bed adult plastic surgery/burn unit (PSBU) at a tertiary care teaching hospital. Between November 11 and December 23, 1996, six additional cases were identified at a community hospital. One of the community hospital cases was transferred from the PSBU. All strains were identical by pulsed-field gel electrophoresis. MRSA may have contributed to skin graft breakdown in one case, and delayed wound healing in others. Patients required 2 to 226 isolation days.

CONTROL MEASURES

A hand held shower and stretcher for showering in the hydrotherapy room of the PSBU were culture positive for the outbreak strain, and the presumed means of transmission. Replacement of stretcher showering with bedside sterile burn wound compresses terminated the outbreak. The PSBU was closed to new admissions and transfers out for 11 days during the investigation. Seven of 12 patients had effective decolonization therapy.

CONCLUSION

Environmental contamination is a potential source of nosocomial MRSA transmission on a burn unit. Notification among institutions and community care providers of shared patients infected or colonized with an antimicrobial resistant microorganism is necessary.

Antimicrobial resistance--pharmacological solutions

The interaction between microbial resistance and antibacterial agents occurs in a direct and an indirect fashion. Directly--through the development of resistance to the agent used, or to agents of the same class--as exemplified by the induction of beta-lactamase by both gram-positive and gram-negative bacteria. It also takes place through the development of resistance to compounds of different classes to the compound used, as exemplified by the loss of Streptococcus pneumoniae susceptibility to penicillin that is accompanied by a parallel loss of sensitivity to erythromycin and to tetracycline. As for the indirect way--microbial resistance may develop through selection of resistant organisms when the patient is treated with antibiotics, when the environment is contaminated with antibiotics (hospital) or when antibacterial agents are used in agriculture and animal husbandry.

Outbreak of infection in a burns unit due to Pseudomonas aeruginosa originating from contaminated tubing used for irrigation of patients

Five patients with extensive deep burns developed septicaemia due to Pseudomonas aeruginosa serogroup O-7.8 and phage type 21 or 21/188 shortly after they had been admitted to hospital. Four other burned patients became colonized with the same strain. The source of infection was contaminated tap water used for irrigation of the burns, as part of the first-aid treatment which the patients received when entering the hospital. Contamination was restricted to showers and tubing that were permanently connected to the taps, and the outbreak stopped after they had been disinfected. Tubing and showers used for irrigation of burns should be dismantled and heat-disinfected after each patient and not reconnected to the taps until immediately before the next treatment. Taps used for irrigation of burns should be monitored regularly for the presence of P. aeruginosa and other potentially pathogenic bacteria. Routine typing of P. aeruginosa isolates from burned patients is indicated in order to detect and eliminate hidden sources of infection.

Epidemic Pseudomonas aeruginosa serotype O16 bacteremia in hematology-oncology patients

From 1 August 1978 through 31 December 1982, 98 hematology-oncology patients had positive cultures for Pseudomonas aeruginosa serotype O16; 22 of these patients developed bacteremia, and this bacteremia was associated with the occurrence of extensive perineal cellulitis in 10 patients (45.5%). Seventeen bacteremic patients died. The epidemic strain differed from other P. aeruginosa organisms isolated at the hospital by its resistance to all antibiotics available at that time (ticarcillin, piperacillin, azlocillin, tobramycin, ceftizoxime, ceftriaxone, moxalactam, ceftazidime, and fosfomycin). Univariate analysis showed the following factors to be significantly associated with P. aeruginosa O16 bacteremia: the severity of granulocytopenia at the time of the bacteremia, more days with fever, the administration of ticarcillin or an aminoglycoside, the receipt of a greater number of antimicrobial agents for a longer period of time before documentation of the bacteremia, and the occurrence of cellulitis. Logistic regression analysis showed that duration of fever, duration of bacteremia, and the number of antimicrobial agents administered before documentation of the bacteremia were the best predictors of P. aeruginosa O16 bacteremia. In a prospective study of the acquisition of P. aeruginosa by hematology-oncology patients, 1,149 specimens (throat and rectal swabs) from 270 patients and 201 specimens from their washbasin drains were collected. On only three occasions was the epidemic strain isolated from both the patient and his or her washbasin, but in each case the colonization of the patient preceded the isolation of the strain from the washbasin. The transmission of any P. aeruginosa organism from washbasin drain to patient could not be documented. Contact isolation precautions from the Centers for Disease Control were used for all hematology-oncology patients colonized or infected with P. aeruginosa after 7 January 1983. No case of P. aeruginosa O16 bacteremia has occurred at Hotel Dieu since July 1984.

Surveillance of aminoglycoside resistance. European data

The susceptibility patterns of gram-negative aerobic organisms to aminoglycosides differ widely from one European health care center to another and depend upon local antibiotic prescribing policies. Reports of the susceptibility of Pseudomonas aeruginosa to gentamicin and tobramycin have ranged from as low as 49.8 percent and 77.7 percent, respectively, in Greece, to as high as 96.6 percent and 99.2 percent, respectively, in the United Kingdom. The susceptibility of P. aeruginosa to gentamicin, tobramycin, and amikacin decreased in our hospital from 73.1 percent, 94.8 percent, and 95.6 percent, respectively, in 1982, to 43.1 percent, 70.6 percent, and 74.3 percent, respectively, in 1984. A prospective surveillance study of the susceptibility of gram-negative aerobic bacilli to four aminoglycosides (gentamicin, tobramycin, amikacin, and netilmicin) was performed over a period of 17 months. Gentamicin and tobramycin were freely used, while the use of amikacin was restricted throughout the hospital during a four-month baseline period (May through August 1984). Gentamicin and tobramycin accounted for 94 percent of the aminoglycoside use. During the following 13 months (September 1984 through September 1985), amikacin was used as the first-line aminoglycoside and accounted for more than 97 percent of the aminoglycoside usage. A total of 1,866 organisms were analyzed during the baseline period; 5,429 were analyzed during the amikacin-usage period. The overall susceptibility to gentamicin, tobramycin, amikacin, and netilmicin increased from 86.9 percent, 90.4 percent, 94.2 percent, and 88.3 percent, respectively, to 92.3 percent, 94.0 percent, 97.3 percent, and 92.3 percent, respectively. P. aeruginosa isolates had the most striking changes, with the susceptibility to gentamicin, tobramycin, amikacin, and netilmicin increasing from 43.1 percent, 70.6 percent, 74.3 percent, and 50.6 percent, respectively, during the baseline period, to 64.5 percent, 81.6 percent, 90.8 percent, and 65.1 percent, respectively, during the amikacin-usage period. The use of amikacin as a first-line aminoglycoside, while use of the other aminoglycosides was restricted, seemed to have a favorable influence on the susceptibility pattern of gram-negative aerobic isolates in our hospital.

Infections in burned patients

Prevention and treatment of burn wound infection requires knowledge of the epidemiology of such infections. Prevention of infection rests on removal of reservoirs or sources of microorganisms from the burn patient's environment and interruption of transfer of microorganisms to the surface of the wound. When prevention fails and burn wound infection develops, successful therapy may depend on an understanding of the epidemiology of the burn wound during therapy. Contrary to the oft stated concept that antibiotics penetrate the avascular burn wound poorly, our study demonstrated that gentamicin and tobramycin achieved therapeutic concentrations in burn wound tissue. As in other types of infections, susceptible microorganisms were eradicated and resistant microorganisms persisted. Of most importance was the observation that resistant microorganisms may repopulate the wound within four days of starting therapy. It would appear that failure of therapy is not due to failure of antibiotics to penetrate the burn wound but rather to rapid development of superinfection during therapy.

Hand carriage of aerobic Gram-negative rods by health care personnel

A quantitative culture technique (hand washed in a glove containing broth for 30 s) was used to determine the frequency of hand carriage of aerobic Gram-negative rods by various groups of health care workers and 104 control subjects. Overall, 31% of health care workers carried aerobic Gram-negative rods on their hands compared to 59% of control subjects (P < 0.001). Enterobacter agglomerans accounted for 40% of the isolates, and other Enterobacter spp. 7%. Other organisms included Acinetobacter calcoaceticus 21%, Serratia spp. 11%, Klebsiella spp. 10%, Moraxella spp. 3%, Pseudomonas spp. 3%, Proteus spp. 1.5%, Escherichia coli 1%; Morganella morganii, Citrobacter freundii, Aeromonas sp. and an isolate that was not speciated accounted for 0.5% each. We conclude that endemic hand carriage of aerobic Gram-negative rods by health care personnel is common, but significantly less than that of control subjects. Enterobacter agglomerans is found so frequently on the hands of control subjects that it must be considered part of the normal hand flora.

Changing patterns of Pseudomonas aeruginosa antibiotic sensitivity

Using a sample of 71 Pseudomonas aeruginosa infected burns patients admitted to the Burns Injury Unit of the Royal Brisbane Hospital within an 11-year period the pattern of sensitivity of the organism to 18 antibiotics was studied longitudinally looking at first and last cultures (either pus, blood or sputum) separately. Only 5 antibiotics reflected a significant change (towards resistance) in sensitivity patterns (chloramphenicol, gentamycin, kanamycin, tetracycline, achromycin). Six of the other 13 antibiotics showed a trend towards increasing resistance but the changes were not statistically significant (polymyxin B, carbenicillin, sulphanomides, cotrimoxazole, streptomycin, teramycin). The other 7 antibiotics showed no change, all but one (colistin) being resistant throughout.

A comparative study of polyantibiotic and iodophor ointments in prevention of vascular catheter-related infection

Using a semiquantitative technique for culturing material from vascular catheters, we studied by random allocation the efficacy of three regimens for site care of 827 catheters used in adult patients: an iodophor ointment (PI2), ointment containing polymyxin, neomycin and bacitracin (PNB), and use of no topical agent whatsoever (control). Even though this is the largest study of this subject, there was not a sufficient number of catheter-related septicemias to permit valid comparisons (two in each group, 0.7 percent). However, the rate of local catheter-related infection (greater than or equal to 15 CFU on semiquantitative culture), the prelude to related septicemia, was significantly lower in the PNB group (2.2 percent, P = 0.02) as compared with controls (6.5 percent). Use of PI2-treated catheters resulted in one-half fewer infections (3.6 percent) than use of control catheters (P = NS). Staphylococcal infections occurred with 15 control catheters, eight treated with PI2 and two with PNB (P = 0.002). Infections by gram-negative bacilli occurred less frequently in both treatment groups than in controls, but three of four Candida infections, including one septicemia, occurred in the PNB group. Topical antimicrobial agents confer modest benefit in protection against catheter-related infection, primarily for peripheral venous catheters that must remain in place for more than four days. If an ointment is to be used, topical PNB may be preferable for peripheral venous catheters and PI2 ointment for central venous catheters used for parenteral nutrition and for arterial catheters.

In vitro studies on netilmicin, a new aminoglycoside antibiotic

Netilmicin, a semisynthetic derivative of sisomicin, was tested in vitro against 600 clinical bacterial isolates. At a concentration of 1.56 mug/ml, over 90% of gram-negative bacilli were inhibited. Netilmicin was substantially more active against isolates of Serratia marcescens and Enterobacter spp. than gentamicin, sisomicin, tobramycin, or amikacin. Isolates of Staphylococcus aureus (both penicillin G susceptible and resistant) were quite susceptible to netilmicin. Most isolates of Klebsiella spp. and Serratia spp. and some of the isolates of Pseudomonas aeruginosa that were resistant to gentamicin proved to be susceptible to netilmicin.

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.

Prophylactic antibiotic in elective gynaecological and obstetrical major surgery

A prospective controlled study of 580 patients who underwent non-radical major gynaecological or obstetrical surgery examined the use of prophylactic antibiotics. The test group of patients (290) showed a significantly lower (P=0.01) incidence of the total complications and the non-infective complication rate as against those among controls (290). The duration of hospitalisation was significantly reduced for both vaginal and abdominal surgery in the test group of patients as against those in the controls. There was no evidence of increased incidence of bacterial resistance or superinfection. The authors recommend the use of prophylactic antibiotic on a short term basis (for a total duration of 4 days), commencing preoperatively to achieve a significant reduction in the infective postoperative morbidity and a shorter stay by the patients.

Gentamicin in ophthalmology

Gentamicin, widely used in the treatment of ocular infections, is discussed in terms of its chemical structure, effectiveness against various organisms, and pharmacology. Dosages and methods of administration are detailed and toxic effects, reversible and irreversible, are noted.

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.

Microbial development of drug resistance: mechanisms and clinical significance

Bacteria have demonstrated a disconcerting ability to develop resistance to antimicrobial agents nearly as quickly as new compounds become available. During the past two decades the molecular bases of several types of resistance have been elucidated. Mechanisms of resistance include the transference of genetic material either through conjugation (involving direct contact between microorganisms), or indirectly through transduction (involving bacteriophages). In addition to this "infectious" drug resistance, genetic mutations which permit the utilization of new metabolic pathways, and the production of enzymes which can inactivate the antimicrobic have been described. One particularly complex problem has been the ability of many Enterobacteriaceae to develop resistance to multiple antimicrobials simultaneously. The possible effect of such an occurrence is illustrated by the recent epidemic of multiply resistant Salmonella typhi in Mexico. Because the typhoid bacilli shared an identical resistance pattern to an epidemic Shigella dysenteriae type 1 the in vivo interspecies transmission of resistance has been postulated. Understanding the various mechanisms of resistance development should allow more rational use of antimicrobial agents.

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.

A quarter-century of the antibiotic era

A series of 1,500 strains of Pseudomonas aeruginosa was collected from a variety of sources to provide a group of strains which would truly represent the species. All of them were pyocine typed, and a wide range of types was included among the isolates from each source. The gentamicin, carbenicillin, colistin, and polymyxin minimal inhibitory concentration of each strain was measured by the agar dilution method employing the Steers inocula replicator. Over 99% of strains were inhibited by 8 μg of gentamicin per ml, by 256 μg of carbenicillin per ml, and by 4 μg of colistin per ml. The small number of strains more resistant to each antibiotic comprised a variety of different pyocine types. Few strains were found to be susceptible to tetracycline, chloramphenicol, kanamycin, or streptomycin at the single concentration tested.

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.

In vitro studies of BB-K8, a new aminoglycoside antibiotic

BB-K8, an aminoglycoside antibiotic which is a derivative of kanamycin, was tested in vitro against 466 clinical bacterial isolates. Over 90% of gram-negative bacilli, except Proteus spp., were inhibited by 3.12 mug of BB-K8 per ml. This antibiotic was consistently more active than kanamycin but less active than tobramycin or gentamicin. Unlike kanamycin, BB-K8 was active against Pseudomonas aeruginosa. Eleven of 19 isolates resistant to either gentamicin or tobramycin, or both, were susceptible to BB-K8.

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.

Tobramycin: in vitro activity and comparison with kanamycin and gentamicin

The in vitro activity of the aminoglycoside antibiotic tobramycin was demonstrated by broth dilution and single-disc methods on 50 isolates each of Staphylococcus aureus, Klebsiella or Enterobacter, indole-positive and -negative Proteus, Escherichia coli, and Pseudomonas aeruginosa. All organisms were inhibited by 6.25 mug or less of the drug/ml. Pseudomonas strains resistant to kanamycin or gentamicin or both were susceptible to tobramycin. Those strains which were inhibited by 6.25 mug of tobramycin/ml by the broth dilution method had zone diameters of 16 mm or more by the single-disc method. Of 313 organisms tested by the disc method, 3 strains were found to be resistant to tobramycin, 73 were resistant to kanamycin, and 18 were resistant to gentamicin. Tobramycin was found to have satisfactory in vitro activity against many clinically important organisms, including strains resistant to gentamicin and kanamycin.