Inhibitory effect against pathogenic and spoilage bacteria of Pseudomonas strains isolated from spoiled and fresh fish.

The antibacterial effects of 209 Pseudomonas strains isolated from spoiled iced fish and newly caught fish were assessed by screening target organisms in agar diffusion assays. One-third (67 strains) inhibited the growth of one or several of six target organisms (Escherichia coli, Shewanella putrefaciens, Aeromonas sobria, Pseudomonas fluorescens, Listeria monocytogenes, and Staphylococcus aureus), of which S. aureus and A. sobria were the most sensitive. The inhibitory action was most pronounced among the strains producing siderophores, and the presence of iron eliminated the antibacterial effect of two-thirds of the inhibitory strains. Siderophore-mediated competition for iron may explain the inhibitory activity of these strains. All but nine of the inhibiting strains were found to inhibit the growth of 38 psychrotrophic S. putrefaciens strains isolated from spoiling fish and fish products. Siderophore-containing Pseudomonas culture supernatants inhibited growth of S. putrefaciens, as did the addition of iron chelators (ethylenediamine dihydroxyphenylacetic acid [EDDHA]). In particular, Pseudomonas strains isolated from newly caught and spoiled Nile perch (Lates niloticus) inhibited S. putrefaciens. This suggests that microbial interaction (e.g., competition or antagonism) may influence the selection of a microflora for some chilled food products.     

Production and specificity of poly- and monoclonal antibodies raised against Shewanella putrefaciens

B. FONNESBECH, H. FRØKIAER, L. GRAM AND C. MOSBY JESPERSEN. 1993. Polyclonal antibodies were raised in rabbits and mice against Shewanella putrefaciens. Murine monoclonal antibodies were produced against the type strain (ATCC 8071) as well as wild type strains isolated from fish products. The specificities of four polyclonal and 12 monoclonal antibodies were tested by dot-blotting, an indirect and a competitive ELISA against 16 Gram-negative strains; including six strains of S. putrefaciens and one strain of Pseudomonas rubescens (NC 10695). All polyclonal antibodies reacted strongly with S. putrefaciens and with Ps. rubescens and cross-reacted with the nine other bacteria ( Pseudomonas spp., Aeromonas spp. and Vibrio anguillarum ). The monoclonal antibodies could be divided into three groups with different patterns of specificity. The largest group (8 monoclonal antibodies) reacted strongly with S. putrefaciens and with Ps. rubescens and showed only weak reactions with the other strains. The results confirm that Ps. rubescens should be classified as S. putrefaciens.     

A Numerical Taxonomic Study of Pseudomonas-like Bacteria Isolated from Fish in Southeastern Queensland and their Association with Spoilage

Pseudomonas -like bacteria isolated from fresh and spoiling fish in southeastern Queensland were subjected to a wide range of physiological and nutritional tests. The results of these tests, together with those of 20 named strains, were analysed numerically, resulting in the formation of 11 groups. Most of the isolates clustered into group 1 and group 2 which also contained the bulk of the strains able to produce spoilage odours when grown in a tryptic digest of fish muscle at 2°C. Almost all of the group 1 organisms produced sulphydryl type odours, had only 50 mol % G + C and were identified as strains of Alteromonas putrefaciens which were deficient in the ability to produce H₂S detectable in Peptone Iron Agar. Certain of the group 2 strains produced fruity and sulphydryl type odours, but these organisms were not distinguishable from other strains in this group not producing odours. Group 2 strains were highly related to Pseudomonas fragi and were intermediate in properties between Ps. fluorescens and Ps. putida. The remaining nine minor groups contained few organisms able to produce spoilage odours.     

Classification of the spoilage flora of fish, with special reference to Shewanella putrefaciens

One hundred and fifty-nine Gram-negative strains isolated from refrigerated fish, taken from the Baltic Sea or Swedish inland waters, together with 32 reference strains of Shewanella, Pseudomonas, Aeromonas and Alcaligenes, were phenotypically classified using 124 unit characters. Data were processed by the Simple Matching (SSM) and Jaccard (SJ) coefficients, and unweighted pair group algorithm with arithmetic averages. Fourteen clusters were defined at the 75% SJ similarity level which correspond to the SSM level of 86%. SJ-based clusters containing field strains were designated Pseudomonas fragi (cluster 1; 31% of the field strains), Ps. lundensis (cluster 2; 2% of the field strains), Ps. fluorescens biovar III (cluster 4; 4% of the field strains), Ps. putida biovar A (cluster 5; 3% of the field strains), Ps. fluorescens/putida (clusters 3 and 6; 6% of the field strains), Psychrobacter (clusters 8 and 9; 3% of the field strains), Shewanella putrefaciens (clusters 10, 11, 12 and 13; 44% of the field strains) and Aer. sobria (cluster 14; 6% of the field strains, all isolated from fresh water fish). Each field strain represented the spoilage flora of refrigerated fish at a total aerobic count of about 10(8) cfu/g. Phenotypic characteristics of major clusters are given. The four S. putrefaciens clusters may be separated by key characteristics. Shewanella putrefaciens ATCC 8071T and reference strains from sources other than fish, did not group in any of the clusters. The mol % guanine + cytosine content was on average 47.6 for cluster 10, and 45.3 for cluster 13.     

Classification of the spoilage flora offish, with special reference to Shewanella putrefaciens

One hundred and fifty-nine Gram-negative strains isolated from refrigerated fish, taken from the Baltic Sea or Swedish inland waters, together with 32 reference strains of Shewanella, Pseudomonas, Aeromonas and Alcaligenes , were phenotypically classified using 124 unit characters. Data were processed by the Simple Matching (S_SM) and Jaccard (S_J) coefficients, and unweighted pair group algorithm with arithmetic averages. Fourteen clusters were defined at the 75% S_J similarity level which correspond to the S_SM level of 86%. S_J-based clusters containing field strains were designated Pseudomonas fragi (cluster 1; 31% of the field strains), Ps. lundensis (cluster 2; 2% of the field strains), Ps. fluorescens biovar III (cluster 4; 4% of the field strains), Ps. putida biovar A (cluster 5; 3% of the field strains), Ps. fluorescens/putida (clusters 3 and 6; 6% of the field strains), Psychrobacter (clusters 8 and 9; 3% of the field strains), Shewanella putrefaciens (clusters 10, 11, 12 and 13; 44% of the field strains) and Aer. sobria (cluster 14; 6% of the field strains, all isolated from fresh water fish). Each field strain represented the spoilage flora of refrigerated fish at a total aerobic count of about 10⁸ cfu/g.
Phenotypic characteristics of major clusters are given. The four S. putrefaciens clusters may be separated by key characteristics. Shewanella putrefaciens ATCC 8071^T and reference strains from sources other than fish, did not group in any of the clusters. The mol % guanine + cytosine content was on average 47.6 for cluster 10, and 45.3 for cluster 13.     

Characterization of Nonfermentative Nonfastidious Gram Negative Bacteria encountered in Medical Bacteriology

More than 90 morphological and physiological characters of 546 strains of nonfermentative, nonfastidious, Gram negative bacteria isolated from clinical specimens were examined to determine those features most useful for the identification of these bacteria. The species examined included Moraxella osloensis, Mor. lacunata, Acinetobacter anitratum, Ac. haemolyticus subsp. haemolyticus, Ac. haemolyticus subsp. alcaligenes, Ac. Iwoffi, Alcaligenes faecalis, Alc. odorans var. viridans, Pseudomonas fluorescens, Ps. putida, Ps. pseudomallei, Ps. maltophilia, Ps. stutzeri, Ps. acidovorans, Ps. alcaligenes , and atypical strains of Ps. aeruginosa.     

Decrease in respiration activity related to prodigiosin synthesis in Serratia marcescens

Variation in the cell respiration rate of pigmented and nonpigmented strains of Serratia marcescens was exhibited. The respiration rate of a pigmented strain decreased earlier than that of nonpigmented strains in the late exponential or early stationary phase. However when prodigiosin synthesis was not induced by exchange of carbon sources in the medium, the decrease in the respiration rate of the pigmented strain was the same as that of nonpigmented strains. Measurement of the oxygen consumption rate in the sonicated cell membrane by adding NADH solution showed that the rate in the pigmented strain was lower than that in nonpigmented strains. Furthermore, the cell membrane of prodigiosin-induced organisms was more sensitive to respiration inhibitors than that of pigment-noninduced organisms of the pigmented strain. These results showed that the respiration activity was decreased by prodigiosin synthesis in S. marcescens.     

Factors contributing to the survival of poultry associated Pseudomonas spp. exposed to a quaternary ammonium compound

Resistance to benzalkonium chloride (BC) among Pseudomonas spp. isolated from poultry carcasses was determined and strategies for elimination of resistant strains evaluated. This investigation showed that resistance was quite common, about 30% of the isolates being able to grow in 200 μg ml⁻¹ BC. Pseudomonas fluorescens strains were generally less susceptible than strains of Ps. lundensis and Ps. fragi. An overnight incubation in medium containing 200 μg ml⁻¹ BC was sufficient to reduce the susceptibility of two Pseudomonas strains to the lethal effect of BC significantly. Adding EDTA enhanced the lethal effect of BC, but the effect was reduced after growing cells in medium containing BC and EDTA. Growth in medium with a quaternary ammonium compound (QAC) rendered the cells more susceptible to chlorine, phenolics and alkylaminoacetate. These results indicate that alternating use of QACs with these compounds can be used to avoid build-up of resistant strains. In addition, increased temperatures improved the lethal effect of BC and should be considered when planning disinfection routines.     

Effect of temperature on the microbial flora of herring fillets stored in air or carbon dioxide

The microbial development on fillets of herring from the Baltic Sea was studied at temperatures from 0-15 C in air or 100% carbon dioxide (96-100% CO2). The shelf-life of the fillets , defined as the time for the 'total aerobic count' to reach 1 X 10(7) c.f.u./g increased with decreased temperature from 1 d at 15 degrees C to 7 d at 0 degrees C (air). The corresponding values in CO2 were 3 d and 33 d, respectively. The initial flora of the herring fillets was dominated by Alteromonas putrefaciens and Pseudomonas spp. and so was the spoilage flora after storage in air (together 62-95% of the flora: all temperatures). Alteromonas putrefaciens predominated slightly at 2 degrees C to 15 degrees C, while Pseudomonas spp. dominated at 0 degrees C. The Pseudomonas flora was mainly split between Ps. fragi , Ps. fluorescens and a heterogenous group of unidentified Pseudomonas spp. The proportions were not influenced by temperature. In 100% CO2 at the time of spoilage the flora consisted of a significant number of Lactobacillus spp. Below 4 C the domination was almost complete while at 10 degrees C and 15 degrees C. Enterobacteriaceae, Vibrionaceae and Alt. putrefaciens was also found. It was concluded that the microbiological shelf-life of herring fillets is improved by refrigeration storage in 100% CO2 but for good results the temperature should not exceed 2 degrees C.     

Cross-reactivity of antibodies raised to Pseudomonas fluorescens protease with extracellular proteins produced by meat-spoiling pseudomonads

The cross-reactivity patterns of antibodies to Pseudomonas fluorescens protease with the extracellular proteins produced by a number of meat-spoiling pseudomonads were studied. Immunoblotting studies showed that purified IgG to Ps. fluorescens protease cross-reacted with extracellular proteins in the cell culture supernatant fluids of Pseudomonas spp., including Ps. fragi and Ps. lundensis. In the case of Ps. lundensis and Pseudomonas spp. 11390, the cross-reactive moieties were of similar molecular weight to the Ps. fluorescens protease (46 kDa). However, in Ps. fragi the cross-reactive moiety was a lower molecular weight protein (8 kDa). This may represent a fragment of the active enzyme. These results indicate the presence of common antigenic determinants among the proteases of meat spoiling pseudomonads.     

Quantitative real-time PCR study of the influence of probiotic culture soluble fraction on the expression of Pseudomonas aeruginosa quorum sensing genes

The opportunistic pathogen Pseudomonas (Ps.) aeruginosa causes severe infections, particularly in immunocompromised individuals and patients with cystic fibrosis (CF). A serious side effect of antibiotic therapy in Ps. aeruginosa infections is the development of resistance to antibiotics. During the infection process Ps. aeruginosa forms biofilms, rendering bacterial cells more resistant to disinfectants, antibiotics and the action of host immune defense effectors. Pseudomonas aeruginosa employs the intercellular communication system, known as quorum sensing (QS) to coordinate the expression of tissue-damaging factors. Since the QS systems controls the production of different virulence factors, it is possible that the inhibition of its regulatory activity to severely compromise the ability of Ps. aeruginosa to cause infections in humans. Many studies have shown that some probiotic strains exhibit inhibitory activity on different virulence properties of pathogenic bacteria (adherence to cellular or inert substrate, soluble virulence factors expression). The aim of the present study was to investigate by real-time RT-qPCR the influence of probiotic culture soluble factors on the QS genes expression in 30 Ps. aeruginosa strains isolated from patients hospitalized in the National Institute for Cardiovascular Infections, Prof. C.C. Iliescu Fundeni Hospital, Bucharest. The results of the real time RT-qPCR have shown that in all Ps. aeruginosa strains grown in the presence of probiotic culture sterile filtrates, the level of QS genes expression was reduced comparatively with those from control cultures. In conclusion, these results proved that the inhibition of virulence factors regulation mechanisms by soluble molecules secreted by probiotics could represent an interesting way pathogenicity and virulence attenuation in Ps. aeruginosa nosocomial strains.     

广州儿童医院重症监护病房感染病原菌的分布及耐药性分析

目的探讨儿科重症监护病房（PICU）感染病原菌的分布及耐药情况,为临床合理选用抗菌药提供参考。方法对广州市儿童医院PICU病房2003年11月-2005年10月各类感染标本所分离的病原菌的分布及耐药性进行回顾性分析。结果共检出295株病原菌,其中革兰阴性杆菌213株（72．2％）,主要为铜绿假单胞菌、不动杆菌等非发酵菌;革兰阳性球菌58株（19．7％）,主要为葡萄球菌;真菌24株（8．1％）。药敏结果提示铜绿假单胞菌及不动杆菌对亚胺培南、头孢哌酮／舒巴坦、环丙沙星及阿米卡星较为敏感,铜绿假单胞菌对头孢噻肟耐药率较高,而不动杆菌对头孢哌酮、氨曲南、庆大霉素耐药严藿。肠杆菌科细菌对氨苄西林、氨苄西林／舒巴坦、哌拉西林及多种头孢菌素耐药率较高而对亚胺培南、头孢哌酮／舒巴坦、阿米卡星等较敏感。葡萄球菌对青霉素、红霉素严重耐药,但对万古霉素、替考拉宁及阿米卡星敏感性高。结论铜绿假单胞菌等非发酵菌已成为PICU病房感染的主要病原菌。根据病原菌种类及药敏结果合理应用抗菌药是有效控制危重病患儿感染和减少耐药菌株产生的重要手段。     

Preliminary description of biocidal (syringomycin) activity in fluorescent plant pathogenic Pseudomonas species

Strains representing the fluorescent plant pathogenic Pseudomonas spp., Ps. agarici , Ps. asplenii , Ps. avellanae , Ps. beteli , Ps. caricapapayae , Ps. cichorii , Ps. corrugata , Ps. ficuserectae , Ps. flectens , Ps. fuscovaginae , Ps. marginalis , Ps. meliae , Ps. savastanoi , Ps. syringae , Ps. tolaasii and Ps. viridiflava were tested for biocidal activity using Aspergillus niger as assay organism. Inhibitory behaviour was found in strains of Ps. asplenii , Ps. blatchfordae , Ps. cichorii , Ps. corrugata , Ps. fuscovaginae , Ps. marginalis , Ps. marginalis pv. pastinacea , Ps. syringae pv. syringae , Ps. syringae pv. aptata , Ps. syringae pv. atrofaciens , Ps. syringae pv. lapsa , Ps. tolaasii , and strains of a Pseudomonas sp. pathogenic to Actinidia , in the Ps. savastanoi genomic sp. Antifungal activity could be identified with the production of members of the syringomycin family of toxins by strains in Ps. syringae , Ps. asplenii and Ps. fuscovaginae . These toxin reactions support suggestions made elsewhere of the synonymy of the latter two species. In a preliminary characterization using tests for stability to heat, protease, acid and alkaline treatments, unknown toxins consistent with syringomycin-like toxins the strains from Actinidia speciesColour RGB 0,0,128. The toxins from Ps. cichorii and from Ps. corrugata differed in their reactions from all other agents. Pseudomonas tolaasii produces the antifungal compound tolaasin. The white line reaction with ' Ps. reactans ', a test for tolaasin production by strains of Ps. tolaasii , was confirmed as specific for this compound. Some of these low molecular weight toxins may be produced by some of these plant pathogenic strains.     

A numerical taxonomic study of the Pseudomonas flora isolated from poultry meat

Pseudomonas strains were isolated from both fresh and cold-stored broiler skin. Phenotypically-based numerical taxonomic techniques were used to characterize the isolates and 36 reference strains. For this purpose, Biolog GN Microplates, API 20NE and a number of other biochemical tests were used. Jaccard clustering revealed the predominance of four major Pseudomonas groups: Ps. fragi, Ps. lundensis, strains belonging to Ps. fluorescens biovars and an unidentified group of strains displaying a high degree of similarity to Ps. fluorescens biovars. Within Ps. fluorescens, biovar A was best represented. The marked proteolytic character of members of Ps. fluorescens biovars A, B and C, as well as of members of the unidentified cluster, supports their possible role in the origin of organoleptic defects. In the Ps. lundensis cluster, a distinct group of Ps. lundensis-like species was found. Further genotypic studies should be carried out to clarify the taxonomic status of the Ps. lundensis-like strains and that of the unidentified group resembling Ps. fluorescens biovars A and B.     

Extracellular Nuclease Activity of Fish Spoilage Bacteria, Fish Pathogens, and Related Species

The production of extracellular deoxyribonuclease and ribonuclease by 23 marine and 3 dairy strains of Pseudomonas putrefaciens, 15 strains of fish-pathogenic fluorescent pseudomonads, 38 strains of fluorescent pseudomonads isolated from haddock, and 34 related organisms was determined by an agar plate method. All strains of P. putrefaciens produced both deoxyribonuclease and ribonuclease. Of the other 87 organisms examined, 26.5% produced ribonuclease and 14.5% produced deoxyribonuclease. All organisms which produced deoxyribonuclease also produced ribonuclease. Deoxyribonuclease production by P. putrefaciens is suggested as a useful criterion of identity for members of this intense fish spoilage species.     

Interaction between fish spoilage bacteria Pseudomonas sp. and Shewanella putrefaciens in fish extracts and on fish tissue

The interaction between fish spoilage bacteria, Pseudomonas sp. and Shewanella putrefaciens , was investigated using fish extract and fish tissue as model systems. Isolates of Pseudomonas that produced iron chelators, siderophores, inhibited growth of S. putrefaciens in a fish-extract-agar diffusion assay but no, or only weak, antagonistic activity was seen when the medium was supplemented with iron. Sterile-filtered supernatant fluid from a siderophore-producing Pseudomonas grown in fish extract was inhibitory to S. putrefaciens if the number of Pseudomonas was above 10⁸ cfu ml⁻¹. In contrast, supernatant fluids from siderophore-negative Pseudomonas isolates did not inhibit growth of S. putrefaciens. The inhibitory effect was, except for one strain of Pseudomonas , not seen in supernatant fluids from iron-enriched cultures of Pseudomonas sp. Finally, siderophore-producing Pseudomonas sp. lowered the maximum cell level of S. putrefaciens 1–2 log units from 10⁹ to 10¹⁰ cfu g⁻¹ when the strains were grown on fish muscle blocks at 0°C but the growth rate of S. putrefaciens was not affected.     

Phenotypic heterogeneity of Pseudomonas corrugata strains from southern Italy

Fifty-five strains of Pseudomonas corrugata isolated in southern Italy were characterized phenotypically and compared with 23 strains of different origins. At least two main cultural types with rough or smooth colonies were observed. Strains with rough colonies produced a diffusible pigment in culture. On the basis of their nutritional profiles, Ps. corrugata strains formed a distinct phenon most closely related to fluorescent Pseudomonas spp. isolated from tomato pith necrosis-diseased plants. Three major groups of strains were differentiated within the Ps. corrugata phenon on the basis of utilization of 2-ketogluconate, meso-tartrate, hystamine, DL- glycerate and induction of a hypersensitive reaction on tobacco. Some Ps. corrugata strains belonging to group 1 and 3 which did not produce pigment in culture produced IAA in a colorimetric test. Variability in the serological reaction of the Italian strain was observed. None of the three antisera utilized reacted with all strains. Some strains isolated from diseased plants from the same greenhouse showed different nutritional profiles and reacted with different antisera. Fifteen lipopolysaccharide (LPS) patterns were observed. Strains were divided into two groups on the basis of their protein profiles. The heterogeneity which had already been observed in a world-wide study on Ps. corrugata was confirmed in strains from this restricted area.     

Characterization of CMR5c and CMR12a, novel fluorescent Pseudomonas strains from the cocoyam rhizosphere with biocontrol activity

AIM: To screen for novel antagonistic Pseudomonas strains producing both phenazines and biosurfactants that are as effective as Pseudomonas aeruginosa PNA1 in the biocontrol of cocoyam root rot caused by Pythium myriotylum. MATERIAL AND RESULTS: Forty pseudomonads were isolated from the rhizosphere of healthy white and red cocoyam plants appearing in natural, heavily infested fields in Cameroon. In vitro tests demonstrated that Py. myriotylum antagonists could be retrieved from the red cocoyam rhizosphere. Except for one isolate, all antagonistic isolates produced phenazines. Results from whole-cell protein profiling showed that the antagonistic isolates are different from other isolated pseudomonads, while BOX-PCR revealed high genomic similarity among them. 16S rDNA sequencing of two representative strains within this group of antagonists confirmed their relatively low similarity with validly described Pseudomonas species. These antagonists are thus provisionally labelled as unidentified Pseudomonas strains. Among the antagonists, Pseudomonas CMR5c and CMR12a were selected because of their combined production of phenazines and biosurfactants. For strain CMR5c also, production of pyrrolnitrin and pyoluteorin was demonstrated. Both CMR5c and CMR12a showed excellent in vivo biocontrol activity against Py. myriotylum to a similar level as Ps. aeruginosa PNA1. CONCLUSION: Pseudomonas CMR5c and CMR12a were identified as novel and promising biocontrol agents of Py. myriotylum on cocoyam, producing an arsenal of antagonistic metabolites. SIGNIFICANCE AND IMPACT OF THE STUDY: Present study reports the identification of two newly isolated fluorescent Pseudomonas strains that can replace the opportunistic human pathogen Ps. aeruginosa PNA1 in the biocontrol of cocoyam root rot and could be taken into account for the suppression of many plant pathogens.     

医院内4种主要革兰阴性菌分布及耐药率分析

摘要：目的 分析大连某三甲医院4种主要革兰阴性病原菌的分布及细菌耐药性变迁,为抗菌药物的合理使用提供依据。方法 回顾性分析2012?2015年住院患者中分离的4种革兰阴性菌临床分布及每年的耐药率,利用WHONET 5.6软件进行统计分析。结果 4年分离致病菌28 485株,其中大肠埃希菌2 994株,肺炎克雷伯菌1 375株,鲍曼不动杆菌1 079株,铜绿假单胞菌1 998株,共占分离致病菌总数的26.1%。4种菌对哌拉西林的耐药率最高（37.2%~78.2%）,对头孢哌酮/苏巴坦耐药率最低（6.7%~39.1%）。2012?2015年,大肠埃希菌、肺炎克雷伯菌和铜绿假单胞菌的抗生素耐药率有下降趋势,鲍曼不动杆菌对多数抗生素耐药率都高,且逐年上升。结论 本院4年感染中4种主要革兰阴性菌占主导地位。4种菌对不同抗生素耐药率和变化不同,鲍曼不动杆菌耐药最严重。4种菌对于头孢哌酮/苏巴坦均比较敏感,对哌拉西林耐药严重。在临床治疗中应有针对性加强管理和用药。     

Frequency of antibiotic and heavy metal resistance, pigmentation, and plasmids in bacteria of the marine air-water interface.

A field investigation of marine coastal waters revealed that the frequency of pigmented bacteria and the occurrence of bacterial antibiotic resistance were higher at the air-water interface than in the bulk water. The differences in the frequency of pigmented colonies at the surface and in the bulk-water samples could not be explained by the degree of cell surface hydrophobicity or by bacterial adhesion to air-water interfaces. Pigmented strains exhibited a higher degree of multiple drug resistance than did nonpigmented strains. However, the frequency of multiple drug resistance in nonpigmented strains was also substantial. An average of 91% of all strains were resistant to more than one antibiotic, and 21% of the bacteria isolated were resistant to five of the eight antibiotics tested. High numbers of plasmid-carrying strains were found among selected surface isolates, but the presence of detectable plasmids could not be correlated with either pigmentation or multiple drug resistance. Furthermore, selected surface isolates were significantly more resistant to mercury than were bulk-water bacteria. The higher frequency of pigmented, antibiotic-resistant, and mercury resistant strains at the air-water interface than in the bulk water are discussed in terms of various forms of selective pressure and genetic exchange at the surface.