Endemic aminoglycoside resistance in gram-negative bacilli: epidemiology and mechanisms

Isolates of gentamicin-resistant gram-negative bacilli from clinical specimens peaked at nine to 10 per month in 1973-1974. Instituting barrier-type precautions during 1974-1977 was associated with a sustained 87% reduction in resistant Enterobacteriaceae. The number of resistant Pseudomonadaceae fell temporarily by 28%, paralleling gentamicin usage. During an endemic 15-month period in 1976-1977 nonenzymatically mediated resistant Pseudomonas aeruginosa often emerged after aminoglycoside therapy in patients who had prior carriage of sensitive strains of the same serotype (P = 0.002); this resistance was associated with wound or sputum isolates (P = 0.003). Resistant Enterobacteriaceae more often demonstrated the converse, that is, spread of urinary tract isolates with enzymatically mediated resistance from patients not on aminoglycoside therapy. These findings suggest that control measures to minimize occurrence of resistant bacilli include barrier-type precautions for patients with resistant Enterobacteriaceae, evaluation of transfers and readmissions as a source of resistant organisms, and reduction of aminoglycoside use to decrease the selection of nonenzymatic resistance.     

Antibiotic rotation and development of gram-negative antibiotic resistance

To attain a better understanding of antibiotic cycling and its effects on the epidemiology of antibiotic resistance in gram-negative microorganisms, two different antibiotic classes (quinolone and beta-lactam) were cycled during four 4-month periods in a surgical intensive care unit. Respiratory aspirates and rectal swabs were obtained and DNA fingerprinting was performed. Primary endpoint of the study was the acquisition rate with gram-negative bacteria resistant to the antibiotic of choice during each cycle. Secondary endpoints were changes in endemic prevalence of resistant bacteria and the relative importance of cross-transmission. In all, 388 patients were included and 2,520 cultures analyzed. Adherence to antibiotic protocol was 96%. Overall antibiotic use increased with 24%. Acquisition rates with resistant bacteria were highest during levofloxacin exposure (relative risk [RR] 3.2; 95% confidence interval [CI]: 1.4-7.1) and piperacillin/tazobactam exposure (RR 2.4; 95% CI 1.2-4.8). The relative importance of cross-transmission decreased during the study. For individual patients, treatment with levofloxacin was the only independent risk factor for acquisition of levofloxacin-resistant bacteria (hazard ratio 12.6; 95% CI 3.8-41.6). Potential for selection of antibiotic-resistant gram-negative bacteria during periods of homogeneous exposure increased from cefpirome to piperacillin/tazobactam to levofloxacin. Cycling of homogeneous antibiotic exposure is unlikely to control the emergence of gram-negative antimicrobial resistance in intensive care units.     

Minimal antibiotic concentrations of aminoglycosides and beta-lactam antibiotics for some gram-negative bacilli and gram-positive cocci.

The minimal antibiotic concentration (MAC) is the lowest concentration of an antibacterial agent that produces a decrease of 1 log in the number of organisms/ml as compared with a control culture in drug-free medium. Various gram-negative bacilli and gram-positive cocci were grown in the presence of amikacin, gentamicin, tobramycin, ampicillin, amoxicillin, oxacillin, carbenicillin, ticarcillin, and cefamandole at concentrations varying from eight times the minimal inhibitory concentration (MIC) to 1/128 of the MIC. Colony forming units (cfu) were counted, the MIC was determined, and the MIC:MAC ratio, which indicates the magnitude of the effective range, was calculated. The MIC:MAC ratio appears to be characteristic for a given species and antibiotic. There is no relation between the MICs and the MIC:MAC ratios. The highest ratios were given by Proteus mirabilis with aminoglycosides (MIC:MAC mean, 29.2 with tobramycin), and the lowest ratios were given with beta-lactam antibiotics by Pseudomonas aeruginosa and Streptococcus faecalis (MIC:MAC means, 2.1 with carbenicillin and cefamandole, respectively).     

Impact of antibiotic resistance on the treatment of gram-negative sepsis

Resistance among gram-negative organisms has greatly complicated the care of the septic patient. An understanding of the likely source of infection, the epidemiologic risk of the patient being exposed to an antibiotic-resistant organism, and the specific vulnerabilities of the host are essential to the proper selection of empiric antimicrobial therapy. In this report, we discuss the epidemiology, antibiotic resistance mechanisms, microbiology, treatment strategies, and diagnostic and therapeutic innovations in the approach to the septic patient.     

Beta lactamase resistance of newer cephalosporins and antimicrobial effectiveness against gram-negative bacilli

Summary Three newer cephalosporins (cefamandole, cefoxitin and cefazaflur) were investigated, in comparison with three older agents (cephalothin, cephaloridine and cefazolin) to determine their stability to -lactamases of gram-negative bacilli, and to correlate this with their antibacterial activity. Nine of the 17 bacterial strains employed produced broad-spectrum -lactamases; the remaining eight produced cephalosporinases. The cephalosporins were highly active against bacteria producing broad-spectrum -lactamases; they were less active against organisms producing cephalosporinases. All of the cephalosporinase-producing strains were resistant to cephalothin and cephaloridine. With the other cephalosporins the correlation between hydrolysis by cephalosporinases and resistance of the organisms was poor. Four of eight cephalosporinase-producing strains were resistant to cefoxitin, which was completely resistant to hydrolysis by the -lactamases. Cefozolin, cefamandole and cefazaflur inhibited several of these strains in spite of destruction by the -lactamase. Several cephalosporins need to be used in antimicrobial susceptibility testing of gram-negative bacilli.

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Beta-Lactamase-Resistenz der neuen Cephalosporine und die antimikrobielle Wirksamkeit gegen gramnegative Bazillen

Zusammenfassung Drei neue Cephalosporine (Cefamandol, Cefoxitin und Cefazaflur) wurden im Vergleich zur drei älteren Präparaten der Cephalosporinreihe (Cephalothin, Cephaloridin und Cefazolin) untersucht, um ihre -Lactamase-Stabilität gegenüber gram-negativen Bazillen abzugrenzen und das Ergebnis mit ihrer antibakteriellen Aktivität zu korrelieren. Neun der 17 verwendeten Stämme produzierten Breitspektrum--Lactamasen. Die restlichen acht produzierten Cephalosporinasen. Die Cephalosporine waren hoch aktiv gegen breitspektrum--lactamase-produzierende Bakterien; sie waren weniger aktiv gegen Keime, die Cephalosporinase bildeten. Alle cephalosporinase-produzierenden Keime waren cephalothin-und cephalosporidin-resistent; bei den anderen Cephalosporinen war die Korrelation zwischen der Hydrolyse durch Cephalosporinasen und der Bakterienresistenz gering. Vier der acht cephalosporinase-produzierenden Stämme erwiesen sich als cefoxitin-resistent. Cefoxitin war jedoch komplett resistent gegenüber der -Lactamasehydrolyse. Cefazolin, Cefamandol und Cefazaflur hemmten einige dieser Stämme trotz der Zerstörung durch die -Lactamasen. Es wird postuliert, daß verschiedene Cephalosporine für die Empfindlichkeitstestung gram-negativer Bazillen erforderlich sind.

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This work was supported by grants from the Lilly Research Laboratories and Smith Kline and French Laboratories.     

The red menace: Emerging issues in antimicrobial resistance in gram-negative bacilli

Gram-negative bacilli cause more than one third of all nosocomial infections in US hospitals. Despite a surfeit of new and highly potent antimicrobial agents, the problem of resistance in these pathogens continues to increase. Particularly important is the emergence of resistance to the fluoroquinolone and β-lactam classes of antimicrobial agents. Recent work has confirmed that resistance to fluoroquinolone antibiotics is a complex process that involves mutations in the target enzymes (topoisomerase II and IV), decreased access to the target enzyme resulting from low permeability of the outer membrane (this is primarily important in Pseudomonas aeruginosa), and active efflux from the cell. Resistance to β-lactam antibiotics, however, is primarily caused by the elaboration of an ever-growing number of β-lactamases. Our ability to understand the genetic and biochemical underpinnings of these resistance phenotypes will be an important factor in determining the ultimate success of efforts to control their emergence and spread.     

The significance of serum-sensitive bacilli in gram-negative bacteremia.

Clinical findings from 76 patients (median age 67 years) with gram-negative bacteremia were analysed and related to the sensitivity of the blood isolates to the bactericidal activity of normal human serum. 28 strains (37%) were resistant, an equal number intermediately sensitive and 20 markedly sensitive (26%). No correlation was found between serum sensitivity and origin of the bacteremia, presence of fever or blood granulocyte count. The frequency of shock in immunocompromised patients with serum-resistant strains was 60% (6/10); in those with intermediately or markedly sensitive strains it was 44% (8/18). In the non-immunocompromised patients with resistant strains the frequency of shock was 33 (6/18) versus 10% (3/30) in those without such strains. Thus the risk of developing shock with gram-negative bacteremia seems to depend on both parasite and host factors, although in this study only the latter were statistically significant. We conclude that serum-sensitive strains can invade the blood stream in spite of the serum bactericidal activity and cause severe disease in some patients.     

Gentamicin resistance in gram-negative bacilli: occurrence of modifying enzymes and their influence on susceptibility testing

Among 272 gram-negative bacilli, regarded as strains with reduced susceptibility to gentamicin by the disc test, 156 strains were resistant (greater than or equal to f microgram/ml) by the agar dilution method. Altogether 76% of the 156 strains were cross-resistant to tobramycin, kanamycin, streptomycin and amikacin. Amikacin showed the highest antibacterial activity in vitro. 32 of the resistant strains produced acetylating or adenylylating enzymes, which caused resistance. The production of enzymes was not limited to any genera. Complete cross-resistance to aminoglycoside antibiotics was recorded in 45%, 98% and 22% respectively, of Pseudomonas aeruginosa, Ps. maltophilia and other gram-negative rods. Enzyme-producing strains were not cross-resistant to all aminoglycosides. Changes in MICs and inhibition zones with different inocula among enzyme-producing strains emphasize the importance of standardized inocula in susceptibility testing.     

我院近3年下呼吸道革兰阴性杆菌耐药性监测

目的 调查我院近3年下呼吸道革兰阴性杆菌的耐药情况.方法 使用MIC法对我院下呼吸道感染住院患者的痰液标本中临床分离的926株革兰阴性杆菌进行药敏试验,并用WHONET 5.4软件进行数据分析.结果 926株革兰阴性杆菌中最常见的菌种依次为大肠埃希菌(29.7%)、肺炎克雷伯菌(23.7%)、铜绿假单胞菌(14.3%)、鲍曼不动杆菌(12.1%).大肠埃希菌、肺炎克雷伯菌对多黏菌素B、亚胺培南、美罗培南和咪诺环素保持高度敏感,耐药菌率在10%以内,对阿米卡星、哌拉西林/三唑巴坦、头孢哌酮/舒巴坦、头孢他啶及头孢匹肟的耐药率为30%以内,对其余所检测药物的耐药率均在30%以上.铜绿假单胞菌对多黏菌素B和咪诺环素;而亚胺培南,美罗培南、哌拉西林/三唑巴坦,头孢哌酮/舒巴坦和阿米卡星耐药率低于30%{鲍曼不动杆菌耐药情况比较严重,只有多黏菌素B、头孢哌酮/舒巴坦高度敏感,耐药率在10%以内,对亚胺培南和美罗培南的耐药率在20%以内,对其余所检测药物的耐药率均在30%到60%以上.结论 本研究结果对我院革兰阴性杆菌感染的经验用药治疗有重要参考价值.     

Increasing antimicrobial resistance in gram-negative bacilli isolated from patients in intensive care units

BACKGROUND: We investigated gram-negative bacilli from patients in intensive care units to determine whether antimicrobial resistance was increasing. METHODS: Minimal inhibitory concentrations were determined by broth microdilution on 334 gram-negative bacilli collected in 1990, 1995, and 1998. RESULTS: During the 3 study years, the types of gram-negative bacilli encountered in our intensive care units changed with proportional increases of Pseudomonas sp and decreases of inducible enterics. Dramatic increases in resistance for ceftazidime, cefotaxime, and piperacillin were paralleled between respiratory-tract isolates and inducible enterics. By 1998, ticarcillin was more active than piperacillin against most isolates except Escherichia coli and Klebsiella sp, and most isolates became more resistant to gentamicin and tobramycin. CONCLUSIONS: Continuous changes in the types of gram-negative bacilli and antimicrobial resistance complicate empirical selection of antimicrobials in the intensive care units. These complications will place more emphasis on communication and strategy formations among health care workers (nurses, physicians, laboratorians, and pharmacists) in an effort to treat infections in a timely and effective manner.     

Lethal effect of complement and lysozyme on polymyxin-treated, serum-resistant gram-negative bacilli.

When genetically serum-resistant Escherichia coli, Klebsiella pneumoniae, and Citrobacter freundii, but not Pseudomonas aeruginosa or Proteus mirabilis, were exposed to polymyxin B, they became susceptible to the bactericidal action of normal human and rabbit sera. In constrast, beta-lactam and aminoglycoside antibiotics did not render any serum-resistant bacteria serum-sensitive. Synergy between polymyxin B and the serum bactericidal system could be demonstrated by the addition of polymyxin B to bacteria in vitro, as well as to bacilli in serum from rabbits injected with the antibiotic. Polymyxin B-treated bacteria were killed by normal, lysozyme-depleted, C2-deficient, and hypogammaglobulinemic sera, but not by heated or C6-deficient sera. These findings indicate that polymyxin B-treated bacteria can be killed via the alternative complement pathway. However, C3 and C3b were detected on the surface of serum-resistant E. coli, regardless of whether the bacteria had been treated with polymyxin B. This observation suggests that a change in susceptibility to the alternative complement pathway was not the only explanation for the acquired serum sensitivity. Polymyxin B may also affect a step in the complement sequence beyond the activation of C3, a step that is apparently blocked in serum-resistant gram-negative bacteria.     

老年下呼吸道感染革兰阴性杆菌现状及耐药性监测

目的了解老年下呼吸道感染病原菌分布,革兰阴性杆菌现状及耐药性检测,指导临床用药.方法收集2003年6月至2005年6月老年病房住院并确诊为下呼吸道感染患者的痰标本进行细菌学分离、鉴定,应用NCCLS推荐的纸片扩散确证法检测超广谱β-内酰胺酶（ESBLs）;K-B琼脂扩散法行药敏试验.结果从临床标本中共分离出208株病原菌,其中革兰阴性（G-）杆菌150株（72.1%）,革兰阳性菌33株（14.9%）,真菌25株（12.0%）.150株G-杆菌中,大肠埃希菌41株、铜绿假单胞菌39株、不动杆菌属32株、肺炎克雷伯菌20株.在大肠埃希菌和肺炎克雷伯菌中,产ESBLs 19株,占31.1%.大多数G杆菌对亚胺培南保持较高敏感性.结论老年下呼吸道感染G-菌中以肠杆菌科细菌和非发酵菌为主,致病菌呈多重耐药趋势应引起重视.     

革兰阴性杆菌耐药状况研究——2008中国美罗培南敏感性监测(CMSS)报告

目的监测2008年中国革兰阴性(G-)杆菌的耐药性。方法收集2008年9月至12月国内10家教学医院收集的1057株非重复的G-杆菌。菌株经中心实验室复核后,采用琼脂稀释法测定美罗培南等广谱抗菌药物的最低抑菌浓度。结果10种抗菌药物对于715株肠杆菌科细菌的抗菌活性,敏感性排在前3位的依次为美罗培南(敏感性99.4%)、亚胺培南(敏感性99.3%)、阿米卡星(90.4%)。对于大肠埃希菌和肺炎克雷伯菌,抗菌活性最高的是:美罗培南、亚胺培南(97.9%～100%)、哌拉西林/三唑巴坦(87.2%～97.9%)、阿米卡星(82.3%～92.9%)。对于阴沟肠杆菌、产气肠杆菌、弗劳地枸橼酸菌,活性最高的依次为美罗培南、亚胺培南(97.4%～100%)、阿米卡星(87.2%～91.9%)、头孢吡肟(82.4%～93.5%)。对于铜绿假单胞菌,活性最高的药物为阿米卡星(77.4%)、美罗培南(76%)、哌拉西林/三唑巴坦(70.5%)和亚胺培南(70.5%)。鲍曼不动杆菌对于亚胺培南、美罗培南、头孢哌酮/舒巴坦的敏感性最高,分别为58.6%、57%、49.2%。对于洋葱伯克霍尔德菌,抗菌活性较高的是头孢他啶(77.4%)、美罗培南(75.5%)、哌拉西林/三唑巴坦(69.8%)。结论碳青霉烯类对肠杆菌科仍保持高活性,但鲍曼不动杆菌和铜绿假单胞菌的耐药性增加,尤其是鲍曼不动杆菌对碳青霉烯类的耐药性显著增加,值得关注。     

In vitro aminoglycoside resistance of gram-negative bacilli and staphylococci isolated from blood in Sweden 1980-1984

The in vitro susceptibility to gentamicin, tobramycin, amikacin and netilmicin in septicaemia isolates was followed during 1980-1984 in 6-8 Swedish laboratories. The bacterial distribution was similar over the years and was dominated by Escherichia coli and staphylococci. Resistance to gentamicin was found in 2.3-3.6%, to tobramycin in 1.4-3.4%, to amikacin and netilmicin in 0.5-0.9%. Production of aminoglycoside modifying enzymes was observed among resistant strains.     

Pharyngeal flora in ambulatory alcoholic patients: prevalence of gram-negative bacilli.

The pharyngeal flora of a group of ambulatory alcoholic patients was studied and compared with the pharyngeal flora of a control group. Sixty-eight patients were studied, 34 alcoholics and 28 controls. Of the alcoholic patients, 59% had Gram-negative bacilli in their pharyngeal flora, while 14% of the control group had the same organisms. There were no differences in Gram-positive cocci colonization between the groups. Klebsiella pneumoniae was the most frequent isolate (40%) and the Klebsiella-Enterobacter group accounted for 76% of the isolates. Colonization rates of greater than 10 colony forming units/ml were found in 43% of the alcoholic patients. The high prevalence and higher colonization rates of Gram-negative bacilli in alcoholic patients might explain the higher incidence of Gram-negative bacillary pneumonia among alcoholics.     

Carriage of gram-negative bacilli in young Brazilian children with community-acquired pneumonia.

BACKGROUND: Gram-negative bacilli are not infrequently encountered as etiologic organisms of pneumonia in children in warm-climate countries. OBJECTIVES: To investigate the nasopharyngeal carriage rate and antimicrobial susceptibility patterns of gram-negative bacilli colonizing children with community-acquired pneumonia in Fortaleza, Brazil. METHODS: A single nasopharyngeal specimen was collected from children 2 months to 5 years of age presenting at one of the three children's hospitals in Fortaleza and fulfilling the World Health Organization criteria for pneumonia. Randomly recruited healthy children from public daycare centers and immunization clinics served as controls. RESULTS: The study included 912 children, 482 (53%) with pneumonia and 430 (47%) controls. Aerobic gram-negative bacilli were seen in 79 (16%) of the 482 children with pneumonia and 51 (12%) of the 430 healthy controls. Nonfermentative gram-negative bacilli were seen in 85 (18%) of children with pneumonia and 54 (13%) of healthy controls. Neither gender, nutritional status, season, previous hospital admission nor antibiotic use was associated with carriage with gram-negative bacilli. However, pneumonia was associated with increased carriage, whereas concomitant colonization with Streptococcus pneumoniae or Haemophilus influenzae was associated with decreased carriage with gram-negative bacilli. Only 36% of all Escherichia species and 76% of all Klebsiella isolates were susceptible to cotrimoxazole; 90% of all Acinetobacter species were susceptible to gentamicin. CONCLUSION: Nasopharyngeal carriage with gram-negative bacilli, in particular with Acinetobacter species, is common and associated with a clinical diagnosis of community-acquired pneumonia in children in Fortaleza, Brazil.     

Sources of gram-negative bacilli colonizing the tracheae of intubated patients.

Twenty acutely ill patients requiring prolonged orotracheal intubation were studied to determine the source and progression of gram-negative bacilli colonizing the trachea. Organisms recovered from daily tracheal, hypopharyngeal, and rectal cultures were typed and speciated to identify identical strains at the three sites. All patients acquired gram-negative bacilli in the trachea by day 3 after intubation. Thirty organisms that were not recovered from the tracheal aspirate immediately following intubation were isolated for at least two days some time thereafter. Nine of the 30 colonizing bacteria were Enterobacteriaceae, and all were found in another culture site, usually the hypopharynx, before isolation from the trachea. In contrast, only four of the 21 non-Enterobacteriaceae that colonized the trachea were recovered previously from either the hypopharynx or rectum, a finding which represents a significant difference (P = 0.0002). Quantitation of isolates from the hypopharynx was of no value in predicting subsequent acquisition in the trachea, and the numbers of bacteria recovered from the first positive tracheal specimen were not predictive of subsequent persistence in the trachea.