Gentamicin resistance in Pseudomonas aeruginosa: R-factor-mediated resistance

Evolution of biofilm-forming pathogenic bacteria in the presence of nanoparticles and antibiotic: adaptation phenomena and cross-resistance

Background

Treatment of bacterial biofilms are difficult and in many cases, expensive. Bacterial biofilms are naturally more resilient to antimicrobial agents than their free-living planktonic counterparts, rendering the community growth harder to control. The present work described the risks of long-term use of an important alternative antimicrobial, silver nanoparticles (NAg), for the first time, on the dominant mode of bacterial growth.

Results

NAg could inhibit the formation as well as eradicating an already grown biofilm of Pseudomonas aeruginosa, a pathogen notorious for its resilience to antibiotics. The biofilm-forming bacterium however, evolved a reduced sensitivity to the nanoparticle. Evidence suggests that survival is linked to the development of persister cells within the population. A similar adaptation was also seen upon prolonged exposures to ionic silver (Ag⁺). The persister population resumed normal growth after subsequent passage in the absence of silver, highlighting the potential risks of recurrent infections with long-term NAg (and Ag⁺) treatments of biofilm growth. The present study further observed a potential silver/antibiotic cross-resistance, whereby NAg (as well as Ag⁺) could not eradicate an already growing gentamicin-resistant P. aeruginosa biofilm. The phenomena is thought to result from the hindered biofilm penetration of the silver species. In contrast, both silver formulations inhibited biofilm formation of the resistant strain, presenting a promising avenue for the control of biofilm-forming antibiotic-resistant bacteria.

Conclusion

The findings signify the importance to study the nanoparticle adaptation phenomena in the biofilm mode of bacterial growth, which are apparently unique to those already reported with the planktonic growth counterparts. This work sets the foundation for future studies in other globally significant bacterial pathogens when present as biofilms. Scientifically based strategies for management of pathogenic growth is necessary, particularly in this era of increasing antibiotic resistance.

Graphic abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12951-021-01027-8.

Antibiotic activity of iron-sequestering polymers

Increasing antibiotic resistance has compelled the development of novel antibiotics and adjuvant therapies that enhance the efficacy of existing antibiotics. Iron plays a critical role in bacterial infections, yet the use of iron chelators as adjuvant therapy with antibiotics has yielded highly variable outcomes. Multivalent polymeric materials offer an alternative approach to bind and sequester iron via high avidity interactions. Here, a biomimetic iron-sequestering polymer (PAI-DHBA) was synthesized by modifying side chains of cross-linked polyallylamine (cPAI) with 2,3-dihydroxybenzoic acid (DHBA). PAI-DHBA polymer gels with various DHBA contents showed high iron affinity indices and high selectivity for iron. The polymers showed mild antibiotic properties when used to treat established bacterial cultures. Pretreating culture media with PAI-DHBA polymer, however, removed all detectable iron from media and effectively inhibited the growth of Pseudomonas aeruginosa. In addition, bacterial growth was more susceptible to antibiotics combined with PAI-DHBA. Multivalent polymers that bind and sequester iron, such as PAI-DHBA, offer a promising early intervention or adjuvant to antibiotics.

Multiple Pathways of Genome Plasticity Leading to Development of Antibiotic Resistance

The emergence of multi-resistant bacterial strains is a major source of concern and has been correlated with the widespread use of antibiotics. The origins of resistance are intensively studied and many mechanisms involved in resistance have been identified, such as exogenous gene acquisition by horizontal gene transfer (HGT), mutations in the targeted functions, and more recently, antibiotic tolerance through persistence. In this review, we focus on factors leading to integron rearrangements and gene capture facilitating antibiotic resistance acquisition, maintenance and spread. The role of stress responses, such as the SOS response, is discussed.

Underexpression of ap from R-plasmids in fast-growing Rhizobium species

The presence of the plasmid RP1 in the cells of Rhizobium leguminosarum strains Rld1, 300, and 248, R. phaseoli 1233, R. trifolii strains T1 and 6661, and R. meliloti 4013 was found to appreciably increase bacterial resistance toward kanamycin and tetracycline but not toward ampicillin. The presence of 16 other R-plasmids in R. leguminosarum was also found to either not increase or only marginally increase bacterial resistance toward ampicillin. It appears now that underexpression of the plasmid-specified ampicillin function is common to most fast- and slow-growing rhizobia.

Class 1 integron containing a new gene cassette, aadA10, associated with Tn1404 from R151

The carbenicillin, gentamicin, kanamycin, streptomycin, spectinomycin, sulfonamide, and tobramycin resistance determinants found on Pseudomonas aeruginosa plasmid R151 have previously been shown to translocate to another plasmid, R388, and it was inferred that a transposon, Tn1404, carried the resistance determinants. Sequencing of the cassette array from the plasmid known as R388::Tn1404 revealed two known gene cassettes, oxa10 and aadB, and a previously unidentified cassette determining resistance to streptomycin and spectinomycin, here designated aadA10, in the order oxa10-aadB-aadA10. These cassettes replaced the dfrB2-orfA cassette array of R388, indicating that movement of the resistance determinants from R151 to R388 resulted from recombinational exchange between two class 1 integrons rather than transposition. The AadA10 protein is most closely related to AadA6 (85% identical) and AadA7 (80% identical). The aadA10 cassette found here has only a simple site containing a 7-bp spacer derived from attI1 in place of a 59-base element and is likely to represent a derivative of the complete cassette. IntI1-mediated deletion of the aadA10 cassette was not detected, indicating that this single simple site is either inactive or only weakly active.

Effect of the IncP-2 plasmids pMG1 and pMG2 on the pyocins of Pseudomonas aeruginosa

Pyocin typing may be unstable under certain circumstances. In the early seventies, it was shown that the IncP-2 plasmids pMG1 and pMG2 interfered with the pyocin type of some Pseudomonas aeruginosa strains. Up to now, details of the influence of these plasmids on the pyocins are unknown. In this study, the effect of these plasmids on the R, F, and S pyocins, respectively, was examined. In distinct strains, S pyocin activity was detected in the presence of these plasmids, whereas R and F pyocin activities could not be observed. So the effect of the IncP-2 plasmids pMG1 and pMG2 may be limited to the R and/or F pyocins.

Aminoglycoside antibiotics in clinical use

Aminoglycosides are among the most used antibiotics despite competitive pressure from newer beta-lactam agents. The activity profile, pharmacology, toxicity potential, and methods of toxicity prevention of aminoglycosides are well appreciated after three decades. Nephrotoxicity, ototoxicity, and the added costs of drug level monitoring limit wider usage, but great activity against highly antibiotic resistant gram negative bacteria often outweigh these disadvantages and will likely keep aminoglycosides available for the foreseeable future.

Sequence of PSE-2 beta-lactamase

The nucleotide sequence of PSE-2 beta-lactamase, an enzyme that readily hydrolyzes both carbenicillin and oxacillin, has been determined. The deduced sequence of 266 amino acids contained 93 residues identical to those of OXA-2 beta-lactamase and the Ser-Thr-Phe-Lys tetrad also found in the active site of TEM-1 beta-lactamase.

Molecular hybridization versus isoelectric focusing to determine TEM-type beta-lactamases in gram-negative bacteria

Isoelectric focusing and molecular hybridization with a TEM DNA probe were used to screen for TEM beta-lactamase in 328 bacterial isolates representing 11 gram-negative genera. The TEM enzyme was detected in 50% of isolates, and nine additional types of beta-lactamase could be identified in 36.9% of isolates. The TEM gene was detected in 53.6% of isolates. The results obtained by both methods were concordant in 92.7% of the entire sample. In situ colony hybridization with a specific probe therefore appears to be a convenient method to screen rapidly for the presence of homologous genetic sequences among a large number of isolates. Positive hybridization was observed for 16 isolates in which no TEM beta-lactamase was detected by isoelectric focusing. The significance of this hybridization remains to be determined.

Mechanisms of resistance to cephalosporin antibiotics

Cephalosporins, like other beta-lactams, bind to the bacterial penicillin-binding proteins (PBPs). These correspond to the D-ala-D-ala trans-, carboxy- and endo-peptidases responsible for catalysing the cross-linking of newly formed peptidoglycan. Resistance arises when the PBPs-and particularly the transpeptidases-are modified, or when they are protected by beta-lactamases or 'permeability barriers'. Target-mediated cephalosporin resistance can involve either reduced affinity of an existing PBP component, or the acquisition of a supplementary beta-lactam-insensitive PBP. beta-lactamases are produced widely by bacteria and may be determined by chromosomal or plasmid DNA. The chromosomal beta-lactamases are species-specific, but can be classified into a few broad groups. The plasmid-mediated enzymes cross interspecific and intergeneric boundaries. The level of beta-lactamase-mediated resistance relates to the amount of enzyme produced with or without induction; to the location of the enzyme (extracellular for Gram-positive organisms and periplasmic in Gram-negative ones); and to the kinetics of the enzyme's activity. In Gram-positive organisms the PBPs are located on the outer aspect of the cytoplasmic membrane and so shielding by permeability barriers is minimal. In Gram-negative cells, however, the PBPs are protected by the outer membrane, which most beta-lactams cross by diffusion through aqueous pores composed of 'porin' proteins. In enterobacteria, a clear correlation exists between porin quantity and cephalosporin resistance, suggesting that the outer membrane is the sole barrier to drug entry. Such relationships are less clear for Pseudomonas aeruginosa, where the cell may contain additional barriers between the outer membrane and the PBPs. Although elevated cephalosporin resistance often is attributed to a single factor (PBP-modification, beta-lactamase action or impermeability) an organism's response to a drug often reflects the interplay of several factors. Mathematical models can be proposed to describe this interplay.

Lipopolysaccharide changes in impermeability-type aminoglycoside resistance in Pseudomonas aeruginosa

Clinical isolates of Pseudomonas aeruginosa were examined for the basis of impermeability-type aminoglycoside resistance. Two apparently related burn isolate strains with high-level (strain 8803) and low-level (strain 13934) gentamicin resistance each had a plasmid. Transformation of the plasmid from either strain to P. aeruginosa PAO503 resulted in low-level gentamicin resistance. No mechanism for this resistance could be determined. Low-level gentamicin and streptomycin resistance from strain 8803 (but not 13934) was transduced with phage E79.tv2 to PAO503 without transfer of plasmid DNA. Transductants like strain 8803 showed absence or reduction of the lipopolysaccharide (LPS) "ladder" pattern of PAO503, had a change in chemical composition of LPS, and, like strain 8803, had a reduced capability to accumulate streptomycin. Comparison of the resistant clinical isolates 8803 and P10 with the apparently related but less-resistant strains 13934 and P10R, respectively, showed the latter strains had LPS ladder patterns and the former strains did not. Strain 8803 had normal outer membrane protein profiles, electron transport components, and transmembrane electrical potential relative to PAO503 and has been previously shown to have no detectable gentamicin-modifying enzymes and normal protein synthesis. We conclude that low-level impermeability-type aminoglycoside resistance in P. aeruginosa results from conversion of smooth LPS to superficial or deeper rough LPS phenotypes. High-level resistance apparently results from a plasmid-specified, but as yet unknown, mechanism combined with the preceding change in LPS structure.

Detection of PSE-2 beta-lactamase in enterobacteria

PSE-2, a plasmid-mediated beta-lactamase which was previously considered pseudomonas specific, was observed in clinical isolates of Escherichia coli, Klebsiella pneumoniae, Providencia stuartii, and Enterobacter cloacae. All four isolates transferred PSE-2 production, together with resistance to gentamicin, tobramycin, kanamycin, and sulfamethoxazole, into Escherichia coli J62-1 and Pseudomonas aeruginosa PU21. Transfer correlated with acquisition by the transconjugants of a 65-megadalton plasmid, and this element was considered to encode PSE-2 expression. The PSE-2 enzyme conferred high-level carbenicillin resistance on both J62-1 and PU21 transconjugants, although the PSE-2-producing Providencia stuartii isolate was susceptible to carbenicillin.

Properties of PSE-2 beta-lactamase and genetic basis for its production in Pseudomonas aeruginosa

The properties of PSE-2 beta-lactamase have been examined by using two new PSE-2-producing plasmids, pMG33 and pMG74, as well as plasmid R151, found in Pseudomonas aeruginosa. PSE-2 beta-lactamase resembled other PSE enzymes in activity against carbenicillin, but it also resembled OXA enzymes, such as OXA-1, in rapid hydrolysis of oxacillin, cloxacillin, and methicillin and in inhibition by sodium chloride but not by cloxacillin. Antisera that inactivated TEM-1, TEM-2, OXA-1, or PSE-1 and PSE-4 beta-lactamase failed to cross-react with PSE-2, which thus appears to be immunologically distinct. The plasmids determining PSE-2 varied in geographical origin, size, transfer proficiency, and incompatibility specificity, but all determined resistance to carbenicillin, gentamicin, kanamycin, streptomycin, spectinomycin, sulfonamide, and tobramycin. From a pUZ8-R151 recombinant plasmid in Escherichia coli, the PSE-2 beta-lactamase gene could be transposed to a second plasmid in a 6.4-megadalton unit together with resistance to gentamicin, kanamycin, streptomycin, spectinomycin, sulfonamide, and tobramycin. Transposition was recA independent. We propose the designation Tn1404 for this unit, which, like transposons carrying OXA-1, PSE-1, PSE-4, and some transposons determining TEM-1, includes genes for beta-lactam, aminoglycoside, and sulfonamide resistance.

Properties of IncP-2 plasmids of Pseudomonas spp

Thirty IncP-2 R plasmids from isolates of Pseudomonas spp. of diverse geographical origins were examined for the production of resistance properties. All the plasmids determined resistance to tellurite and all inhibited the propagation of certain DNA phages, although several patterns of phage inhibition were detected. Of the 30 plasmids, 29 determined resistance to streptomycin, 28 determined resistance to mercuric ion, and 24 determined resistance to sulfonamide. Resistance to other antibiotics, to compounds of arsenic, boron, or chromium, and to UV irradiation was less common. The degradative plasmid CAM also belonged to this group. When CAM was introduced into recipients carrying an IncP-2 R plasmid, recombinant plasmids were often formed in which antibiotic resistance and the ability to grow on camphor were transferred together to further recipients or were lost together in a strain in which IncP-2 plasmids were unstable. Such hybrid plasmid formation was rec dependent. CAM and other IncP-2 plasmids that determine UV light resistance demonstrated UV-enhanced, nonpolarized transfer of the Pseudomonas aeruginosa chromosome. By agarose gel electrophoresis, all IncP-2 R plasmids and CAM were ca. 300 X 10(6) in molecular weight.

Plasmid-mediated gentamicin resistance of Pseudomonas aeruginosa and its lack of expression in Escherichia coli

We isolated 11 nonconjugative plasmids mediating resistance to aminoglycoside antibiotics, including gentamicin, from Pseudomonas aeruginosa strains. Their genetic properties were investigated in both P. aeruginosa and Escherichia coli transformants. The plasmid molecular weights ranged from 11 x 10(6) to 24 x 10(6). A low level or complete absence of gentamicin resistance was observed when these plasmids were introduced into E. coli, but gentamicin resistance was restored when the plasmids were transferred back to P. aeruginosa from E. coli. Aminoglycoside-modifying enzyme activity was detected in P. aeruginosa harboring these plasmids, but was absent or greatly reduced in E. coli strains. This lack of expression may explain the observed decrease in aminoglycoside resistance.

Evidence for a chromosomal site specifying amikacin resistance in multiresistant Serratia marcescens

We studied 21 strains of amikacin-resistant Serratia marcescens from three different U.S. cities, Twenty of the 21 strains contained conjugative R plasmids mediating gentamicin and tobramycin resistance. Amikacin-resistant S. marcescens from two cities predominated in protracted outbreaks. Conversely, the amikacin-resistant Charleston strain (serotype 02/03:nonmotile) was isolated from only four patients during an outbreak of gentamicin- and tobramycin resistant, amikacin-susceptible S. marcescens (serotype O19:O17). Five different representative amikacin-resistant S. marcescens, each containing a single conjugative plasmid, elaborated a nontransferable aminoglycoside (6')-N-acetyltransferase [AAC(6')] with similar substrate profiles in addition to other transferable aminoglycoside-modifying enzymes. One amikacin-resistant S. marcescens cured of its plasmid and another naturally occurring plasmid-free amikacin-resistant S. marcescens elaborated only AAC(6')-1. These data support the concept of a chromosomal locus in S. marcescens for AAC(6')-1 which commonly coexists with plasmid-mediated genes for aminoglycoside-modifying enzymes.

Aminoglycoside dosing in renal transplant patients. Comparison of nomogram and individualized pharmacokinetic methods in patients with shifting renal function

Serum amikacin concentrations were compared in infected renal transplant recipients that were compared in infected renal transplant recipients that were dosed using a creatinine-based nomogram (group I) or an individual computer-assisted pharmacokinetic dosing method (group II). A total of 30 treatment courses were administered. Mean postinfusion peak levels were 22 microgram/ml in group I and 23.4 microgram/ml in group II. Mean serum trough levels were 8.8 microgram/ml and 5.5 microgram/ml in groups I and II, respectively. Both peak and trough serum levels were significantly more often in the acceptable therapeutic (peak 20-32 microgram/ml) and nontoxic (through less than 10 microgram/ml) ranges in group 11 patients. Seventy-seven per cent of group II and 38% of group I peak levels were in the therapeutic range, while 87% of group II and 70% of group I trough levels were less than 10 microgram/ml. Ototoxicity developed with similar frequency in both groups and occurred significantly more often with a peak level greater than 32 microgram/ml. Declining renal function, usually as a result of allograft rejection, occurred in seven (44%) group I and only three (25%) group II patients but could not be exclusively related to amikacin in any patient. A serum trough level of greater than 10 microgram/ml was associated with an increased risk of declining renal function independent of other risk factors. Failures of aminoglycoside therapy are frequently associated with inadequate serum levels. Conversely, ototoxicity and nephrotoxicity may be related to elevated serum aminoglycoside concentrations. For these reasons, the computer-assisted pharmacokinetic dosing method should be used in septic surgical patients whose renal function is subject to sudden and unexpected changes.

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.

Resistance to aminoglycoside antibiotics of gram-negative bacilli isolated in Canadian hospitals

A survey was made of the frequency of resistance to amikacin, gentamicin and tobramycin among aerobic gram-negative bacilli isolated over a 4-week period in 1979 at six large, geographically separated Canadian hospitals. In the entire series of 4407 isolates the frequency of resistance was 2.5% to amikacin, 8.1% to gentamicin, 5.9% to tobramycin and 1.7% to all three. Most (81%) of the resistant bacteria were acquired by the patients after admission to hospital. The frequency of resistance to the three aminoglycoside antibiotics in each hospital largely reflected the local rate of cross-infection by endemic strains of resistant bacteria.

Tn2001, a transposon encoding chloramphenicol resistance in Pseudomonas aeruginosa

We isolated a new transposon, Tn2001, from the group P-2 plasmid Rms159-1 in Pseudomonas aeruginosa. Tn2001-encoded chloramphenicol resistance did not result from the formation of chloramphenicol acetyltransferase. Tn2001 was transposable between temperate phages and conjugative and nonconjugative plasmids belonging to various incompatibility groups, including P-1, P-3, P-4, P-5, P-7, and P-8 in P. aeruginosa. Transposition occurred independently of the general recombination ability of the Pseudomonas host, and its frequency varied between 10(-1) and 10(-8), depending upon the donor and recipient replicons. Tn2001 transposition also occurred in a recombination-deficient strain of Escherichia coli. Agarose gel electrophoresis and electron microscopic observations revealed that Tn2001 could transpose to different sites in the RP4 replicon and that the transposed deoxyribonucleic acid fragment was 2.1 kilobases long.

Bacteriology and treatment of gram-negative pneumonia in long-term hospitalized children

Gram-negative bacillary pneumonia was diagnosed in 39 longterm hospitalized children, ranging in age from 4 months to 14 years (mean seven years). Fifteen had pneumonitis, 18 necrotizing pneumonia, and six lung abscess. An associated empyema was noted in six cases. Specimens for culture were obtained through percutaneous transtracheal aspiration. Five microorganisms were the predominant isolates: P aeruginosa (13 instances), K pneumoniae (11), E coli (7), S marcescens (6), and P mirabilis (2). There were 112 aerobic and 25 anaerobic isolates recovered from the 39 patients, accounting for 3.5 isolates per specimen. Other aerobic and or anaerobic organisms were mixed with the Gram-negative bacilli in half the patients. All patients were treated with gentamicin for 10 to 26 days (average 19.5 days), and all were cured. In 12 patients in whom other organisms resistant to gentamicin were also present, other antimicrobial agents were concomitantly administered.

Characteristics of gentamicin resistance in nosocomial infections

Characteristics of gentamicin resistance were studied in gram-negative bacilli from 50 consecutive patients with nosocomial infection, during a time when gentamicin resistance had recently become prevalent at Medical University Hospital. Burns, decubitus ulcers, and cystic fibrosis were common precipitating factors for acquisition of gentamicin-resistant organisms. Pseudomonas aeruginosa accounted for 76% and Enterobacteriaceae for 24% of isolates. There was high prevalence of cross-resistance to amikacin (61%) and tobramycin (58%). Of the P aeruginosa strains 36% possessed plasmids which were rapidly detected by agarose gel electrophoresis. None of the isolates transferred gentamicin resistance. Representative isolates failed to elaborate aminoglycoside-modifying enzymes or to take up labelled amikacin. Multiple immunotypes of P aeruginosa were identified. These data suggest that a nonplasmid mediated resistance mechanism such as impermeability was responsible for the emergence of gentamicin resistance.

In vitro susceptibility of gentamicin and/or tobramycin resistant gram-negative bacilli to seven aminoglycosides

The in vitro activity of gentamicin, tobramycin, sisomicin, netilmicin, amikacin, kanamycin and streptomycin was tested simultaneously by the agar dilution method against 584 clinical isolates of gram-negative bacilli that were resistant to gentamicin and/or tobramycin. About half of the gentamicin-resistant Pseudomonas were susceptible to tobramycin but cross-resistance was virtually complete between gentamicin and tobramycin for Enterobacteriaceae. Sisomicin was much more active than gentamicin against Klebsiella, Escherichia and Citrobacter species. Only 18.9%, 27.4% and 27.9% of Klebsiella, Enterobacter and Serratia respectively were resistant to netilmicin. Amikacin was the most effective aminoglycoside with an overall resistance of 15.6%. Kanamycin was effective against 40% of Proteus and Providencia species. Surprisingly, more than half of Klebsiella and Enterobacter species and 85.3% of Serratia species were susceptible to streptomycin.

Isolation of TOL and RP4 recombinants by integrative suppression

We obtained genetic and molecular evidence of non-thermosensitive recombinants of RP4 (Kmr Tcr Cbr/Apr) and the thermosensitive TOL plasmid. As first isolated in Pseudomonas aeruginosa PAO, the recombinant plasmid pTN1 specified noninducible synthesis of TOL enzymes and was transmissible to Escherichia coli on selection for the transfer of kanamycin resistance. The phenotypic expression of TOL genes of pTN1 in E. coli was low and also noninducible. A spontaneous segregant, pTN2, appearing from pTN1, conferred inducible synthesis of TOL enzymes. These plasmids carry all of the TOL determinants as evidenced by the ability of Pseudomonas putida carrying recombinant plasmids to grow on toluene, xylene, and m-toluate. In E. coli the expression of TOL genes with normal regulation (pTN2) appears to be extremely low without induction, and the induced expression is comparable to that with defective regulation (pTN1). The measurement of the molecular weight of pTN2 by electron microscopy gave a value of about 74 X 10(6).

Treatment of complicated urinary tract infections with amikacin: comparison of low and high dosage

A randomized prospective study was carried out comparing the recommended dose of amikacin sulfate (Amikin) (15 mg./Kg.) with a lower dose (7.5 mg./Kg.) in the treatment of complicated urinary tract infections. A total of 50 patients were entered in the study over a period of sixteen months. Bacteriologic results were comparable in both groups. Laboratory values remained normal with the exception of 1 patient in the high-dose group who had renal insufficiency. No evidence of ototoxicity was recorded. Our data indicate that low dosage amikacin sulfate is satisfactory in the treatment of complicated urinary tract infections.

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.

Treatment of hospita-acquired infections with amikacin

Amikacin was used in the treatment of 56 serious gram-negative infections in 54 patients of whom 47 survived. In six of the seven nonsurvivors, the infections were under control at the time of death. The clinical isolates were multiple drug-resistant gram-negative organisms, with Proteus rettgeri being most common. Forty-five (80%) of these infections were nosocomial in origin, and the genitourinary tract was the source in 39 (70%). Complications directly related to amikacin therapy were few and suggested renal or otologic toxicity. In this series of patients, amikacin appeared to be of therapeutic benefit in the treatment of serious gram-negative infections.

Amikacin treatment of pulmonary infections involving gentamicin-resistant gram-negative bacilli

Twelve patients with pulmonary infections presumably involving gentamicin-resistant gram-negative bacilli were evaluated for their response to amikacin therapy. All patients had hospital-acquired infections for which they had been previously treated and were considered therapeutic failures with gentamicin or tobramycin. Assessment of response to amikacin therapy showed objective evidence of clinical improvement in 11. The gentamicin-resistant organism was eradicated in nine patients although, in the majority, other gram-negative bacilli persisted in respiratory tract secretions both during and after treatment. There was one clinical failure.

Emergence of gentamicin-resistant Klebsiella in a general hospital

Gentamicin had been in use in a general hospital for over 7 years before any gentamicin-resistant Klebsiella were observed. In 1974 and 1975, nine different gentamicin-resistant serobiotypes of Klebsiella were isolated from 35 patients. The first strain to appear had R-factor-mediated gentamicin resistance, and it infected 19 patients during a period of almost 2 years, spreading largely by case-to-case infection in patients with urinary catheters. It appeared to lose the capacity to transfer its gentamicin resistance after it had infected five of the patients. We had previously isolated on the same ward a gentamicin-susceptible Klebsiella of identical type, and it was found to be capable of acquiring an R-factor for gentamicin resistance. All of the other types of gentamicin-resistant Klebsiella infected few patients and did not persist in the hospital; four of them had R-factor-mediated resistance to gentamicin and all four, as did the original strain, cotransferred kanamycin, neomycin, and tobramycin resistance. Every gentamicin-resistant Klebsiella was susceptible to amikacin and 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.

Classification of R plasmids by incompatibility in Pseudomonas aeruginosa

R plasmids identified in Pseudomonas aeruginosa strains isolated in our laboratories were classified by their incompatibility characters by using intraspecies conjugation in P. aeruginosa. The fertility factor, FP2, was compatible with R plasmids belonging to all groups, and was classified in a new group. From these data, the plasmids examined have been placed into seven or eight incompatibility groups.

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.

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.

Resistance plasmids of Pseudomonas aeruginosa: change from conjugative to nonconjugative in a hospital population

Properties of a population of carbenicillin- and gentamicin-resistant, tobramycin-susceptible Pseudomonas aeruginosa at Veteran's Administration Hospital, Cincinnati, Ohio, have been followed during a 16-month period. As originally described, the strains were isolated from patients with urinary tract colonizations and were predominantly Parke-Davis immunotype 7. For the majority of these organisms, antibiotic resistance was correlated with the presence of a self-conjugative plasmid of incompatibility group P-2. The source and relative incidence of multiply resistant isolates have remained constant during the current study, but the immunotype has shifted form type 7 to type 2. Concomitantly, the population has lost the property of conjugative transfer of resistance, and resistant strains are now compatible with P-2 plasmids. A group P-2 R plasmid, pMG5, will mobilize resistance markers, demonstrating that the multiple resistance of the nonconjugative strains is mediated by R plasmids. Additionally, gentamicin resistance due to either conjugative or nonconjugative plasmids is correlated with the presence of similar gentamicin acetyltransferase activity. pMG5-mobilized plasmids are shown to be incompatible with pMG5. pMG5 is also shown to mobilize resistance markers from nontransferring antibiotic-resistant strains representing populations from Parkland Memorial Hospital, Dallas, Texas, and Cleveland Clinic Foundation, Cleveland, Ohio.

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.

R factor-mediated resistance to aminoglycoside antibiotics in Pseudomonas aeruginosa

Conjugal transferability of drug resistance was examined, in eleven Pseudomonas aeruginosa strains which were isolated in Frankfurt. Four R factors were demonstrated from three strains using P. aeruginosa as recipients but they were nontransferable to Escherichia coli K12. Two R factors, i.e., Rms146 and Rms147, mediated resistances to tetracycline (TC), streptomycin (SM), sulfanilamide (SA), kanamycin (KM), lividomycin (LV), gentamicin C complex (GM) and 3', 4'-dideoxykanamycin B (DKB). They mediated the formation of aminoglycoside-inactivating enzymes, i.e., SM phosphotransferase, SM adenylyltransferase, KM and LV phosphotransferase l, and GM and DKB 6'-N-acetyltransferase. TC resistance conferred by these R factors was due to impermeability of the drug. P. aeruginosa Ps 142 carried two kinds of R factor in one cell, Rms148 (SM) and Rms149 (SM-SA-GM-CPC) (CPC, carbenicillin). Rms148 (SM) was transferable at a high frequency of 10-1 and mediated the formation of SM phosphotransferase. Rms149 mediated the formation of drug-inactivating enzymes, ie., GM 3-N-acetyltransferase and beta-lactamase, but did not inactivate SM. SM resistance was probably due to impermeability of the drug.

Mobilization of plasmid-borne drug resistance determinants for transfer from Pseudomonas aeruginosa to Escherichia coli

RSU2, a plasmid transmissible between strains of P. aeruginosa but not to Escherichia coli can be mobilized by R751. Conjugatants receive a single plasmid composed of DNA from both R751 and RSU2 which has the compatibility properties of a member of group P (like R751). Study of this fusion plasmid suggests that the failure of RSU2 to transfer into enterobacteria is due to an inability to replicate in these bacteria. The fusion plasmid replicates using the genes of R751.

Third type of plasmid conferring gentamicin resistance in Pseudomonas aeruginosa

R1033 is a plasmid of compatibility group P (= P1) transferred from a wild strain of Pseudomonas aeruginosa. It confers resistance to gentamicin by gentamicin acetyl-transferase 1 and to kanamycin and neomycin by neomycin phosphotransferase 1.

RPL11, an R factor of Pseudomonas aeruginosa determining carbenicillin and gentamicin resistance

R factor RPL11 from Pseudomonas aeruginosa determines multiple drug resistance including resistance to gentamicin and carbenicillin. The host range and incompatibility properties of RPL11 are those of incompatibility group P-2. Strains harboring the factor are not altered with respect to the major immunotypes 1 through 7 of Parke-Davis, or with respect to pyocin type by using the 18 indicators of Jones and co-workers. Analytical ultracentrifugation of crude extracts of R factor-containing strains shows a band of satellite DNA with a buoyant density of 1.717 g/cm(3).