The diversity of multi-drug resistance profiles in tetracycline-resistant Vibrio species isolated from coastal sediments and seawater

In this study we examined the multi-drug resistance profiles of the tetracycline (TC) resistant genus Vibrio to determine its susceptibility to two beta-lactams, ampicillin (ABPC), and mecillinam (MPC), as well as to macrolide, erythromycin (EM). The results showed various patterns of resistance among strains that were isolated from very close geographical areas during the same year, suggesting diverse patterns of drug resistance in environmental bacteria from this area. In addition, the cross-resistance patterns suggested that the resistance determinants among Vibrio spp. are acquired differently within the sediment and seawater environments.     

Sensitivity of Vibrio and Aeromonas to antibiotics]

The antibiotic sensitivity of 696 cultures belonging to the family Vibrionaceae (V. cholerae O1, V. cholerae non-O1, V. albensis, V. parahaemolyticus, V. alginolyticus and Aeromonas spp.) was studied and general regularities of the antibiotic sensitivity were shown: a high sensitivity to broad-spectrum antibiotics (tetracycline and chloramphenicol) and a low sensitivity to ++beta lactams (carbenicillin and ampicillin). The comparative examinations revealed similarity in the antibioticograms of V. cholerae O1 (el Tor++), V. cholerae non-O1 and V. albensis, especially the latter two groups, as well as the tested halophilic Vibrio cultures by the range of the MICs, Mo, Me and the nature of the antibiotic resistance. Cultures of V. cholerae and luminescent Vibrio tended to preserve a high sensitivity. High resistance levels were noted in the halophilic Vibrio and Aeromonas cultures. No significant differences in the sensitivity of the strains of various origin (from man and environmental objects) were detected. However, several more resistant strains were isolated from the environmental objects.     

Screening and evaluation of probiotics as a biocontrol agent against pathogenic Vibrios in marine aquaculture

AIMS: The present work aims at finding potential probionts from marine sources as a biocontrol agent against pathogenic Vibrio species in shrimp larval culture. METHODS AND RESULTS: A total of 109 bacterial strains were isolated from seawater, sediment and marine fish-gut samples, and were screened for their antagonistic activity against Vibrio species. Three strains (Q, Q1 and M) isolated from the marine sediment were found antagonistic against Vibrio strains. Based on 16S ribosomal DNA gene sequence analysis, the strain Q was identified as Paenibacillus spp. (EF012164); Q1 as Bacillus cereus (DQ915582); and the M as Paenibacillus polymyxa (DQ915580). Further, the two bacterial species, Paenibacillus spp. and B. cereus were challenged separately at two different concentrations of 10(4) and 10(5) CFU ml(-1) for probiotic activity in the postlarvae of Penaeus monodon against pathogenic Vibrio harveyi and Vibrio spp. CONCLUSIONS: The present study identified the probiotic activity of Paenibacillus spp., B. cereus and Pa. polymyxa against the pathogenic Vibrios in the postlarvae of P. monodon. SIGNIFICANCE AND IMPACT OF THE STUDY: In vivo study reveals that the marine bacterial species can be used as probionts against pathogenic Vibrios in shrimp larval culture practices.     

Antimicrobial resistance in bacteria isolated from aquaculture sources in Australia

AIMS: To carry out a preliminary assessment of the occurrence of resistance to antimicrobials in bacteria that has been isolated from a variety of aquaculture species and environments in Australia. METHOD AND RESULTS: A total of 100 Gram-negative (Vibrio spp. and Aeromonas spp. predominantly) and four Gram-positive bacteria isolated from farmed fish, crustaceans and water from crab larval rearing tanks were obtained from diagnostic laboratories from different parts of Australia. All the isolates were tested for sensitivity to 19 antibiotics and Minimal Inhibitory Concentrations were determined by the agar dilution method. Plasmid DNA was isolated by the alkali lysis method. Resistance to ampicillin, amoxycillin, cephalexin and erythromycin was widespread; resistance to oxytetracycline, tetracycline, nalidixic acid and sulfonamides was common but resistance to chloramphenicol, florfenicol, ceftiofur, cephalothin, cefoperazone, oxolinic acid, gentamicin, kanamycin and trimethoprim was less common. All strains were susceptible to ciprofloxacin. Multiple resistance was also observed and 74.4% of resistant isolates had between one and ten plasmids with sizes ranging 2-51 kbp. CONCLUSIONS: No antibiotics are registered for use in aquaculture in Australia but these results suggest that there has been significant off-label use. SIGNIFICANCE AND IMPACT OF STUDY: Transfer of antibiotic resistant bacteria to humans via the food chain is a significant health concern. In comparison with studies on terrestrial food producing animals, there are fewer studies on antibiotic resistance in bacteria from aquaculture enterprises and this study provides further support to the view that there is the risk of transfer of resistant bacteria to humans from consumption of aquaculture products. From the Australian perspective, although there are no products registered for use in aquaculture, antimicrobial resistance is present in isolates from aquaculture and aquaculture environments.     

Prevalence of tet(B) and tet(M) genes among tetracycline-resistant Vibrio spp. in the aquatic environments of Korea

Of 24 tetracycline(Tc)-resistant Vibrio spp. isolated from different marine sources in Korea between 1993 and 2003, 23 were identified as carrying both tet(B) and tet(M), while 1 strain carried tet(B) only. In conjugation experiments, 3 strains appeared to be able to transfer both tet(B) and tet(M) to the recipient. Both discriminatory PCR and sequence analysis showed that tet(M) genes of Vibrio spp. appear to be a single allele containing a specific region of tet(M) in Tn1545. However, erm(B) and aphA3, known to be linked to Tn1545-like genes, were not detected in Tc-resistant Vibrio spp., even in 9 strains resistant to erythromycin. In analysis to examine the relative position of tet(B) and tet(M), it was shown that tet(M) was present at the 3'-end of the insertion sequence IS10 of Tn10 carrying tet(B). At the junctional region between Tn10 and tet(M), we found a 14 bp sequence of unknown function and the deletion of regulatory sequences reported to be needed for tet(M) expression in conjugative transposons. This is the first report of the simultaneous presence of tet(B) and tet(M), and of the tet(M) gene being linked to the 3'-end of Tn10 in Tc-resistant Vibrio spp. in Korea.     

香味菌临床分离株耐药谱分析

目的回顾性分析本院临床分离的香味菌的药物敏感情况,指导临床合理使用抗生素。方法收集本院2012年1月至2013年12月期间临床分离的香味菌,采用法国生物梅里埃公司的全自动微生物分析仪VITEK-2进行菌株鉴定,以及VITEK-2配套药敏试验复合板进行药敏检测,分析其耐药谱。结果共收集了8株香味菌,且均呈现多重耐药谱。对头孢菌素类抗生素的耐药率很高,特别是对头孢唑林、头孢他啶、头孢曲松的耐药率均为100%。对氨曲南和呋喃妥因的耐药率均为100%。对碳青霉烯类抗生素亚胺培南的非敏感率达到87.5%。而耐药率最低的左旋氧氟沙星为37.5%。结论本院分离的香味菌呈现多重耐药谱,需对其耐药机制进一步研究。     

Characterization of Lactococcus garvieae isolated from radish and broccoli sprouts that exhibited a KG+ phenotype, lack of virulence and absence of a capsule

AIMS: To identify Lactococcus garvieae isolates from radish and broccoli sprouts and compare them with virulent and less virulent mutant strains obtained from yellowtails with regard to KG phenotype, presence of a capsule and virulence towards yellowtails and mice. METHODS AND RESULTS: Comparative 16S rRNA gene sequence analysis of six isolates obtained from radish and broccoli sprouts indicated that they were L. garvieae (similarity >99%). They were compared with KG9502, Lg2 and ATCC49156 strains obtained from yellowtails. A less virulent mutant strain Lg2-S was obtained by Lg2 subculture. Biochemical characterization of the six strains resembled that of KG9502, Lg2, ATCC49156 and Lg2-S, except for saccharose and tagatose acidification and the presence of hippuricase. These six strains were nonpathogenic towards yellowtails and mice, nonsusceptible to bacteriophages and demonstrated heterogeneity on pulsed-field gel electrophoresis analysis. Using transmission electron microscopy, a capsule was observed in KG9502 and Lg2 but not in ATCC49156 and Lg2-S. CONCLUSIONS: We isolated L. garvieae strains that lacked pathogenicity towards yellowtails and mice from radish and broccoli sprouts; these were noncapsulated and exhibited KG(+) phenotype. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first documentation of L. garvieae isolated from terrestrial plants. These isolates exhibited genetic diversity; however, they were noncapsulated and nonpathogenic towards yellowtails and mice.     

Multiple antibiotic resistance of Vibrio cholerae serogroup O139 in China from 1993 to 2009

Regarded as an emerging diarrheal micropathogen, Vibrio cholerae serogroup O139 was first identified in 1992 and has become an important cause of cholera epidemics over the last two decades. O139 strains have been continually isolated since O139 cholera appeared in China in 1993, from sporadic cases and dispersed foodborne outbreaks, which are the common epidemic types of O139 cholera in China. Antibiotic resistance profiles of these epidemic strains are required for development of clinical treatments, epidemiological studies and disease control. In this study, a comprehensive investigation of the antibiotic resistance of V. cholerae O139 strains isolated in China from 1993 to 2009 was conducted. The initial O139 isolates were resistant to streptomycin, trimethoprim-sulfamethoxazole and polymyxin B only, while multidrug resistance increased suddenly and became common in strains isolated after 1998. Different resistance profiles were observed in the isolates from different years. In contrast, most V. cholerae O1 strains isolated in the same period were much less resistant to these antibiotics and no obvious multidrug resistance patterns were detected. Most of the non-toxigenic strains isolated from the environment and seafood were resistant to four antibiotics or fewer, although a few multidrug resistant strains were also identified. These toxigenic O139 strains exhibited a high prevalence of the class I integron and the SXT element, which were rare in the non-toxigenic strains. Molecular subtyping of O139 strains showed highly diverse pulsed-field gel electrophoresis patterns, which may correspond to the epidemic state of sporadic cases and small-scale outbreaks and complex resistance patterns. Severe multidrug resistance, even resistance transfers based on mobile antibiotic resistance elements, increases the probability of O139 cholera as a threat to public health. Therefore, continual epidemiological and antibiotic sensitivity surveillance should focus on the occurrence of multidrug resistance and frequent microbial population shifts in O139 strains.     

Dynamics of drug resistance of Vibrio cholerae isolated from surface water reservoirs in Siberia and the Soviet Far East in 1976-1990]

In the study on antibiotic resistance 1383 strains of El Tor Vibrio cholerae isolated from surface water reservoirs in 12 administrative territories of the Siberia and Far East within a period of 15 years were tested. The following antibiotics were used: ampicillin, streptomycin, monomycin, polymyxin, tetracycline, chloramphenicol, rifampicin and nalidixic acid. The resistance was unstable and its pattern was wave-like according to annual changes in the biological cycle. It was especially evident in regard to ampicillin, streptomycin, monomycin and polymyxin. The highest numbers of the strains were resistant to polymyxin, ampicillin and streptomycin (up to 100 per cent in some years). The lowest numbers of the strains were resistant to chloramphenicol (0.4 per cent) and tetracycline (1.9 per cent). No strains resistant to rifampicin and nalidixic acid were isolated. In some cases the antibiotic resistance level depended on the geographical zone where the strain was isolated. A direct quantitative dependence of the resistance level on the MIC was observed: the lower the MIC of the drug was, the lower the number of the strains resistant to it was. Within the 15-year period there was no general tendency to increase the resistance in V. cholerae to the antibiotics used.     

Inhibition of Vibrio parahaemolyticus by a bacteriocin-like inhibitory substance (BLIS) produced by Vibrio mediterranei 1

AIMS: The aim of this research was to identify and partially purify new bacteriocin-like substances from strains of halophilic 'non-cholera' vibrios isolated from food sources. METHODS AND RESULTS: Forty-five halophilic Vibrio spp. strains were screened for antimicrobial production. Vibrio mediterranei 1, a nonpathogenic strain, showed antimicrobial activity towards Vibrio parahaemolyticus spp. and related species. The bacteriocin-like inhibitory substance (BLIS), released by the bacteria into growth media, was concentrated by ultrafiltration and characterized. BLIS was sensitive to proteinase K, was stable in the pH range 5-9, was resistant to organic solvents and was heat stable up to 75 degrees C. Initial purification of BLIS by size exclusion chromatography showed an apparent molecular mass of 63-65 kDa. CONCLUSIONS: This study reports the ability of V. mediterranei 1 to produce a bacteriocin-like substance inhibiting growth of V. parahaemolyticus spp. and other closely related bacteria. SIGNIFICANCE AND IMPACT OF THE STUDY: The strong activity of BLIS towards the human and fish pathogen V. parahaemolyticus and the persistence of antimicrobial properties under a variety of different conditions suggest its potential application in food microbiology.     

Resistance to antibiotics and heavy metal salts of coagulase-positive staphylococci belonging to different biological types (ecovars)

A total of 189 strains of S. aureus isolated from cows, sheep, swines, poultry, monkeys, rabbits, foxes, and humans and 23 strains of S. intermedius isolated from minks and sables were studied. The staphylococci belonged to different biological types (according to the Hajek-Marsalek's scheme) and ecovars (according to the Meyer-Witte's scheme). The strains were studied with respect to their resistance to 10 antibiotics (benzylpenicillin, streptomycin, tetracycline, levomycetin, erythromycin, oleandomycin, neomycin, kanamycin, monomycin, and novobiocin), mercuric chloride and cadmium sulfate. As a whole the frequency of resistance to the above preparations among the staphylococci of the animal origin was not high. Differences in the frequency and range of the resistance between strains belonging to different biological types (ecovars) were shown. The highest number of the resistant cultures was detected among the strains of the biological type E (ecovar canis) and atypical strains of S. intermedius isolated from the minks. The least number of the resistant cultures was detected among the strains of the biological type C1 (ecovar bovis) isolated from the cows. It was found that almost all strains of S. intermedius were resistant to cadmium sulfate. This may be used as an additional characteristic of the species.     

Conjugation of drug-resistance plasmids from vibrio anguillarum to Vibrio parahaemolyticus

Drug-resistant strains of Vibrio anguillarum, a fish pathogen, were isolated from diseased fish in culture ponds. In investigations of these strains, the transfer of resistance to chloramphenicol, tetracycline, and sulfanilamide from multiple resistant organisms to laboratory recipients was observed. The plasmid from V. anguillarum was stably maintained in both recipient strains of Vibrio parahaemolyticus and Escherichia coli. The plasmids isolated from Vibrio anguillarum belong to incompatibility group C. The molecular weight of these plasmids determined by electron microscopic observation was about 103 to 113 megadaltons.     

Distribution, genetic richness and phage sensitivity of Vibrio spp. from coastal British Columbia

This study examined the distribution, susceptibility to viral infection and genetic diversity of Vibrio spp. in the coastal waters and sediments of British Columbia during summer (July and August). Abundances of presumptive Vibrio spp. ranged from 1.5 to 346 ml(-1) within the water column (1-291 m); whereas, abundances at the water-sediment interface were much higher (up to approximately 3 x 10(4)Vibrio spp. cc(-1)), and decreased with sediment depth (down to 30 cm). The genetic diversity of Vibrio spp. isolates was not tied to the location from which they originated and was only influenced in a minor way by the type of environment. However, the environment had a greater effect on phage-typing patterns. Vibrio parahaemolyticus isolates from environments with high abundances of cells (sediments and oysters) were generally more susceptible to viral infection than those from the water column which were highly resistant. Therefore, although Vibrio spp. were widespread in the areas investigated, the results show that there is segregation of bacterial host strains in different environments, under differing selection pressures, which ultimately will affect in situ phage production.     

Seasonal incidence of autochthonous antagonistic Roseobacter spp. and Vibrionaceae strains in a turbot larva (Scophthalmus maximus) rearing system

Bacteria inhibitory to fish larval pathogenic bacteria were isolated from two turbot larva rearing farms over a 1-year period. Samples were taken from the rearing site, e.g., tank walls, water, and feed for larvae, and bacteria with antagonistic activity against Vibrio anguillarum were isolated using a replica plating assay. Approximately 19,000 colonies were replica plated from marine agar plates, and 341 strains were isolated from colonies causing clearing zones in a layer of V. anguillarum. When tested in a well diffusion agar assay, 173 strains retained the antibacterial activity against V. anguillarum and Vibrio splendidus. Biochemical tests identified 132 strains as Roseobacter spp. and 31 as Vibrionaceae strains. Partial sequencing of the 16S rRNA gene of three strains confirmed the identification as Roseobacter gallaeciensis. Roseobacter spp. were especially isolated in the spring and early summer months. Subtyping of the 132 Roseobacter spp. strains by randomly amplified polymorphic DNA with two primers revealed that the strains formed a very homogeneous group. Hence, it appears that the same subtype was present at both fish farms and persisted during the 1-year survey. This indicates either a common, regular source of the subtype or the possibility that a particular subtype has established itself in some areas of the fish farm. Thirty-one antagonists were identified as Vibrio spp., and 18 of these were V. anguillarum but not serotype O1 or O2. Roseobacter spp. strains were, in particular, isolated from the larval tank walls, and it may be possible to establish an antagonistic, beneficial microflora in the rearing environment of turbot larvae and thereby limit survival of pathogenic bacteria.     

Drug resistance in Vibrio cholerae strains isolated from clinical specimens

Cholera is a serious epidemic and endemic disease caused by the Gram-negative bacterium Vibrio cholerae. SXT is an integrative conjugation element (ICE) that was isolated from a V. cholerae; it encodes resistance to the antibiotics chloramphenicol, streptomycin and sulfamethoxazole/trimethoprim. One hundred seven V. cholerae O1 strains were collected from cholera patients in Iran from 2005 to 2007 in order to study the presence of SXT constin and antibiotic resistance.The study examined 107 Vibrio cholerae strains isolated from cholera prevalent in some Iranian provinces. Bacterial isolation and identification were carried out according to standard bacteriological methods. Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) to four antibiotics (chloramphenicol, streptomycin, sulfamethoxazole, and trimethoprim) were determined by broth microdilution method. PCR was employed to evaluate the presence of established antibiotic resistance genes and SXT constin using specific primer sets.The resistance of the clinical isolates to sulfamethoxazole, trimethoprime, chloramphenicol, and streptomycin was 97%, 99%, 99%, and 90%, respectively. The data obtained by PCR assay showed that the genes sulII, dfrA1, floR, strB, and sxt element were present in 95.3%, 95.3%, 81.3%, 95.3%, and 95.3% of the V. cholerae isolates.The Vibrio strains showed the typical multidrug-resistance phenotype of an SXT constin. They were resistant to sulfamethoxazole, trimethoprime, chloramphenicol, and streptomycin. The detected antibiotic resistance genes included dfrA for trimethoprim and floR, strB, sulII and int, respectively, for chloramphenicol, streptomycin, sulfamethoxazole, as well as the SXT element.     

Detection of R+ Bacteria in Cultured Marine Fish,Yellowtails (Seriola quinqueradiata)

Many drug-resistant gram-negative bacilli were isolated from the intestinal tracts of yellowtails (Seriola quinqueradiata) cultured on farms in various parts of Kochi and Ehime Prefectures, Shikoku Island. They were Vibrio, Pseudomonas, Proteus, Citrobacter and some unidentified Enterobacteriaceae. Of these drug-resistant strains, Vibrio and Pseudomonas were found to carry R factors in high frequencies. These R factors had four types of resistance markers, SA, SA. CM, SM. CM. ABP. and SA. SM. CM. TC. All R factors were found to belong to the fi^– type. In contrast, only one drug-resistant gram-negative bacillus was detected in a cultured yellowtail on farms near Numazu City, Shizuoka Prefecture.     

Emergence of tetracycline resistance due to a multiple drug resistance plasmid in Vibrio cholerae O139

Abstract Of the 173 clinical strains of Vibrio cholerae O139 isolated from India, Bangladesh, and Thailand tested, six strains from India were resistant to tetracycline, ampicillin, chloramphenicol, kanamycin, and gentamicin. These six strains harbored a self-transmissible plasmid that mediated resistance to tetracycline, ampicillin, chloramphenicol, kanamycin, gentamicin, sulfamethoxazole, trimethoprim, and O/129. The multiple drug resistance plasmids were 200 kb in size and belonged to the incompatibility group C. Although a majority of the O139 strains (94.8%) were highly resistant to streptomycin, sulfamethoxazole, trimethoprim, and O/129, the tetracycline-susceptible strains so far tested were plasmid-negative. The data suggest the existence of two distinct multiple antimicrobial agent resistance (MAR) patterns in V. cholerae O139.     

The isolation and characterization of Campylobacter jejuni subsp. jejuni from domestic geese (Anser anser)

AIMS: The objectives of this study were to determine the presence of thermophilic Campylobacter spp. in free range domestic geese, and to characterize isolated strains using phenotyping criteria and SDS-PAGE of whole-cell proteins. METHODS AND RESULTS: Forty cloacal swabs from two different flocks of domestic geese were examined. All Camp. jejuni strains isolated from geese were biotyped using the Lior biotyping scheme. Twelve Camp. jejuni isolates were also tested for their susceptibility to 17 different antibacterial agents by a disc diffusion METHOD: Fourteen of the isolates were also subjected to SDS-PAGE. All of the geese examined were found to harbour Camp. jejuni. Six geese carried more than one species of Campylobacter. All strains examined were susceptible to various antibiotics but resistant to penicillin G and cephalothin. Eleven strains (92%) were resistant to sodium cefuroxime, and eight (67%) were resistant to cloxacillin, ampicillin and colistin sulphate. Three strains (25%) were resistant to tetracycline, and one strain was resistant to sulfamethoxazole/trimethoprim and kanamycin. Nine strains were subtyped as Camp. jejuni subsp. jejuni biotype II and the remaining ones as biotype I. There were 96% and 100% similarities between all the strains examined by SDS-PAGE. CONCLUSION: This study showed that Camp. jejuni were common in the intestinal tract of domestic geese. SIGNIFICANCE AND IMPACT OF THE STUDY: Geese should be considered as potential reservoirs for human and animal campylobacteriosis. The antibiotic resistance data from this study also showed that fluoroquinolone resistance, which appears to be a problem in poultry isolates in some countries, is not yet a problem in these geese.     

Analysis of the etiological structure and dynamics of antibiotic resistance in the causative agents of suppurative inflammatory diseases in the patients at a large general hospital

The structure of the causative agents isolated from patients with pyoinflammatory infections in 1980-1983 was analysed. It was shown that the surgical and urological infections were mainly caused by gram-negative bacteria. The other pyoinflammatory infections were mainly due to gram-positive cocci. A relatively high frequency of the strains of gram-negative bacteria, especially among Pseudomonas spp. and Enterobacter spp., resistant to aminoglycoside antibiotics, such as gentamicin, sisomycin and tobramycin with preserved sensitivity to amikacin and netilmicin in the majority of the strains was shown. Among the beta-lactam antibiotics cephotaxim and cephalotin were most active against gram-negative bacteria and staphylococci, respectively. The majority of the antibiotic resistant strains of gram-negative bacteria had analogous structures and levels of resistance to 7-12 antibiotics which might indicate the occurrence of 1-2 resistance plasmids among the clinical strains.     

香港养殖海鲷弧菌致病菌药物敏感性及耐药质粒研究

从发病海鲷(Sparus sarba)中共分离到51株弧菌(\%Vibrio)\%,经API20E细菌快速鉴定系统及Alsina和Blanch关键生理生化特性分析鉴定为7个种,它们分别是：溶藻胶弧菌(\%V.alginolyticus)(24株),创伤弧菌(V.vulnificus)(12株)和副溶血弧菌(V.parahaemolyticus)(7株),火神弧菌(V.logei)(4株),远洋弧菌Ⅱ菌(V.pelagius Ⅱ)(2株),河弧菌(V.fluvialis)(1株)和地中海弧菌(V.mediterranei)(1株)\%。其中3种优势菌溶藻胶弧菌创伤弧菌和副溶血弧菌证实对海鲷有致病性。另外采用平板稀释法检测了51株菌对16种抗菌素的敏感性。发现所有菌株对ceftriaxone,链霉素,萘啶酮酸和利福霉素敏感,几乎所有菌株对ceftazidime, netilimicin,氯霉素和sulfamethoxazole敏感.大部分菌株对氨苄青霉素 (60.8%),cefuroxime(667%),丁胺卡那霉素(55%),卡那霉素(588%)和三甲氧苄氨嘧啶(765%)等具有较强的耐药性。通过对菌株中所含有的耐药质粒进行分析,发现15株菌株含有1～4个质粒,分子量范围为9～123kb之间,对12株既含有较大分子量质粒又具有耐药性的菌株进行了质粒转化试验,结果其中9株菌的质粒具有转化能力,转化率为１０－１１～１０－９,表明所分离的菌株的抗药性是由于细菌染色体相关突变造成的。