Introduction
Enterobacter cloacae E. cloacae is an important pathogen, especially in nosocomial bacteremia [
1]. Members of the
E. cloacae complex have emerged as increasingly important nosocomial pathogens, especially in intensive care units (ICUs) [
2]. According to a large-scale survey in the USA,
Enterobacter spp. (mainly
E. cloacae) accounted for 3.9% of all nosocomial bloodstream infections ranking the eighth in all microorganisms causing bacteremia [
3]. β-lactamases are bacterial enzymes that are encoded by chromosomal or plasmid-borne genes. The enzymes protect microorganism against the lethal effects of β-lactam antibiotics by hydrolyzing the β-lactam ring [
4]. As more and more β-lactamase antimicrobial agents, including the third and fourth generation cephalosporins, monobactams, carbopenems and the component contained β-lactamase and β-lactamase inhibitors, were used in the clinical therapy which lead to more and more bacteria resist to these drugs. Even worse is there are more multiply resistant strains appearing, treatment of
E. cloacae infection becomes more and more difficult because of limited antimicrobial options. Thus, the prevalence and antimicrobial resistance associated with
E. cloacae were investigated in this study.
Materials and methods
Materials
Seventy-seven strains of E. cloacae were collected from different parts of clinical patients in a hospital of Tianjin from January, 2004 to January, 2005. Escherichia coli ATCC 25922, E. cloacae 029M (high-yield AmpC β-lactamase) and E. cloacae 029 (hyperintroduce AmpC β-lactamase) were used as the reference strains for antimicrobial susceptibility test and phenotype test.
Methods
Species identification was detected by VITEK (Bacterial automatic biochemical analyzer) based on standard biochemical tests. Antibiotic sensitivity test was performed by VITEK as well. Then, 34 strains of high suspicious E. cloacae which were resistant to the second generation cephalosporin were selected to detect their AmpC β-lactamase.
AmpC enzyme was detected by five-disk diffusion test [
5]. There are two principles for judging continuing high-yield expression of AmpC β-lactamase. First, isolate is sensitive to cefepime (FEP) and imipenem (IP) while resistant to cefotaxime (CTX), ceftazidime/clavulanic acid (CD02) and cefotaxime/clavulanic acid (CD03), or scattered antibacterial colonies appear in the antibacterial ring of CD02, CD03 and CTX. Second, isolate is sensitive to IP and resistant to FEP, CTX, CD02, CD03. If tangential phenomenon appears between antibacterial rings of CD02 and CD03, isolate is judged as hyperinducible expression of AmpC β-lactamase. Other phenotypes are judged as no expression of AmpC β-lactamase.
Results
Resistance of Enterobacter cloacae
The resistant effects of E. cloacae to ampicillin and piperacillin are different, the rates of resistance were 70.1% and 29.9%, respectively. The resistances of E. cloacae to the first and the fourth generation cephalosporins were different, including cefazolin (80.5%), cefuroxime (44.2%), ceftazidime (31.2%), cefotaxime (26.0%) and cefepime (20.8%). The resistances to component β-lactamase were different from 13% to 41.6%. Sulperazone has the lowest drug resistance in our survey, with the rate of resistance of 13%. Meanwhile the resistances to amnoglycosides and quinolones were also lower. The results are described in Table 1.
The rate of carrying AmpC β-lactamase
Among 77 strains, 34 strains of high suspicious E. cloacae which were resistant to the second generation cephalosporinwere selectedto be detected were negative of AmpC β-lactamase. The rest 27 isolates were ture positive to AmpC β-lactamase, therefore,The rate of carrying AmpC β-lactamase in the 77 strains of E. cloacae was 35.06 % (27/77) .
Discussion
E. cloacae is a gram-negative bacillus belonging to Enterobacter. It can be found in sewerage, soil, vegetable and milk productions. In most cases, it is a conditional bacterium [
6]. Usually, it is the normal flora in intestine of human and animal. However, it is becoming an important nosocomial pathogen in recent years [
2,
7,
8]. When the patient is in poor condition (severely chronic disease, decrease or deficiency in immune system function, old, trauma operation, using hormone and immunosuppressive agents, etc),
E. cloacae is easy to colonize and proliferate. The infections often occur in patients with foreign devices implanted, and often associated with bacteremia or urinary tract infection or respiration tract infection [
9]. Ying [
6] reported
E. cloacae could be isolated nearly in all parts of the hospital, especially in intensive care unit (ICU). In America, 5%-7% nosocomial bacteremia is caused by
Enterobacter. Data from isolates recovered from ICUs revealed that this organism is the third-most-common pathogen recovered from the respiratory tract, the fourth-most-common pathogen recovered from surgical wounds, the fifth-most-common pathogen recovered from the urinary tract, and the fifth-most-common pathogen recoveredfrom blood [
10].
As the third generation cephalosporins are used in large scale in the recent years, drug resistance of bacteria in nosocomial infection makes physicians headache. Zhu [
11] reported the resistance of
E. cloacae to cefoxitin, Ampicillin and cefazolin all exceeded 80% in China. Compared to the result from Zhu’s report [
11], the resistance rate in our survey is lower than the average level in the whole country. But we also should notice that the resistance of
E. cloacae to Cefazolin, Cefoxitin and Ampicillin are still serious, with resistance rates of 80.5%, 75.3% and 70.1%, respectively. As Table 1 described, it is sensitive to Sulperazone, and the resistant rate of
E. cloacae to Sulperazone 13.0%. So, Sulperazone is more efficient in stably depressed strains and can be used as clinical empirical therapy. Furthermore, the resistance of
E. cloacae to amikacin and ciprofloxacin are relatively lower than 20%. Therefore physician can use these agents to treat
E. cloacae infection. When these two type agents are considered to use in clinical therapy, only one of these two drugs is usually selected. If the effect of one drug is not good, the other one is also useless, so other types of antibiotics must be taken into account [
12,
13].
According to the drug resistance results, the correlation between the
E. cloacae’s resistant phenotype and its resistant mechanism is demonstrated. Many articles reported that
E. cloacae had AmpC β-lactamase, which was a type of cephalosporinase and its gene was usually located in the chromosomes of bacteria. This enzyme degraded cefoxitin and could not be inhibited by clavulanic acid. In our survey, phenotype test showed that 35.06%
E. cloacae produced AmpC β-lactamase. This result is similar to Zhou’s report (24.31%) [
5] and Zhu’s report (30.4%) [
11]. But Zhang [
14], reported 80% isolates of
E. cloacae produced AmpC β-lactamase. Maybe the rising rate of AmpC β-lactamase is associated with different regions and the application of antibiotics in each region and so on.
Apart from this, the worse is that
E. cloacae producing extended-spectrum β-lactamases (ESBLs) has been reported in different countries and regions [
15]. This enzyme hydrolyzed the first, second and third generation cephalosporins [
16-
19]. Simultaneously, increasing reports [
20] recently show that the strains with ESBLs are resist to cefoxitin, which may be the reason to elucidate the higher resistance rate on cefoxitin of
E. cloacae than its AmpC carrying rate. Therefore, the drug resistance of
E. cloacae to antibiotics is becoming more and more serious in the recent years.
In conclusion, the resistance of E. cloacae to antibiotics is becoming increasingly serious. Antimicrobial agents should be applied more reasonably by physicians in order to lower down the resistant strains’ rising rate. Therefore, further epidemiological investigation should be conducted. Moreover,βresearches on the resistant mechanisms ofβE. cloacae should be further developed. Only in this way, the effective measures to control the spread of resistant strains can be established, better cost-effect will be achieved and the social medical resources can be used effectively.
Higher Education Press and Springer-Verlag Berlin Heidelberg
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