Purification of Pseudomonas aeruginosa exoenzyme S

Sources, purification, immobilization and industrial applications of microbial lipases: An overview

Microbial lipase is looking for better attention with the fast growth of enzyme proficiency and other benefits like easy, cost-effective, and reliable manufacturing. Immobilized enzymes can be used repetitively and are incapable to catalyze the reactions in the system continuously. Hydrophobic supports are utilized to immobilize enzymes when the ionic strength is low. This approach allows for the immobilization, purification, stability, and hyperactivation of lipases in a single step. The diffusion of the substrate is more advantageous on hydrophobic supports than on hydrophilic supports in the carrier. These approaches are critical to the immobilization performance of the enzyme. For enzyme immobilization, synthesis provides a higher pH value as well as greater heat stability. Using a mixture of immobilization methods, the binding force between enzymes and the support rises, reducing enzyme leakage. Lipase adsorption produces interfacial activation when it is immobilized on hydrophobic support. As a result, in the immobilization process, this procedure is primarily used for a variety of industrial applications. Microbial sources, immobilization techniques, and industrial applications in the fields of food, flavor, detergent, paper and pulp, pharmaceuticals, biodiesel, derivatives of esters and amino groups, agrochemicals, biosensor applications, cosmetics, perfumery, and bioremediation are all discussed in this review.

Pseudomonas aeruginosa exoenzyme S induces transcriptional expression of proinflammatory cytokines and chemokines

Pseudomonas aeruginosa infection of cystic fibrosis patients causes lung damage that is substantially orchestrated by cytokines. In this study, multi-gene probe analysis was used to characterize the ability of the P. aeruginosa mitogen, exoenzyme S, to induce proinflammatory and immunoregulatory cytokines and chemokines. Exoenzyme S strongly induced transcription of proinflammatory cytokines and chemokines (tumor necrosis factor alpha, interleukin-1alpha [IL-1alpha], IL-1beta, IL-6, IL-8, MIP-1alpha, MIP-1beta, MCP-1, RANTES, and I-309), modest transcription of immunoregulatory cytokines (IL-10 and IL-12p40), and weak transcription of Th1 cytokines (IL-2 and gamma interferon). The response occurred early and subsided without evolving over time. These data suggest that cells responding to exoenzyme S would rapidly express proinflammatory cytokines and chemokines that may contribute to pulmonary inflammation in cystic fibrosis.

Pseudomonas aeruginosa exoenzyme S stimulates murine lymphocyte proliferation in vitro

The exuberant immunoinflammatory response that is associated with Pseudomonas aeruginosa infection is the major source of the morbidity and mortality in cystic fibrosis (CF) patients. Previous studies have established that an exoproduct of P. aeruginosa (exoenzyme S) is a mitogen for human T lymphocytes and activates a larger percentage of T cells than most superantigens, which may contribute to the immunoinflammatory response. An animal model would facilitate studies of the pathophysiologic consequences of this activation. As a first step toward developing an animal model, the murine lymphocyte response to exoenzyme S was examined. When stimulated with exoenzyme S, splenocytes isolated from naive mice entered S phase and proliferated. The optimum response occurred after 2 to 3 days in culture, at 4 x 10(5) cells per well and 5.0 micrograms of exoenzyme S per ml. The response was not due to lipopolysaccharide, since Rhodobacter sphaeroides lipid A antagonist did not block the response. Other preparations of exoenzyme S stimulated lymphocyte proliferation, since the response to recombinant exoenzyme S (rHisExo S) cloned from strain 388 was similar to the response to exoenzyme S from strain DG1. There was evidence that genetic variability influenced the response, since A/J, CBA/J, and C57BL/6 mice were high responders and BALB/cJ mice were low responders following stimulation with exoenzyme S. Both splenic T and B lymphocytes entered the cell cycle in response to exoenzyme S. Thus, murine lymphocytes, like human lymphocytes, respond to P. aeruginosa exoenzyme S, which supports the development of a murine model that may facilitate our understanding of the role that exoenzyme S plays in the pathogenesis of P. aeruginosa infections in CF patients.

RecombinantPseudomonasexoenzyme S and exoenzyme S fromPseudomonas aeruginosaDG1 share the ability to stimulate T lymphocyte proliferation

Exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 have distinct characteristics, which has led to a controversy about their homology and their pathophysiologic consequences. We have been investigating the ability of exoenzyme S to activate T lymphocytes, and therefore performed studies to determine whether exoenzyme S from P. aeruginosa DG1 and recombinant exoenzyme S derived from strain 388 and expressed in Pseudomonas aeruginosa PA103 or in E. coli BL21(DE3), could induce T lymphocyte activation and proliferation. Both preparations were able to activate T cells and induce lymphocyte proliferation at similar levels as measured by flow cytometry of surface-activation markers and DNA synthesis, respectively. Further, a monoclonal antibody raised against exoenzyme S from strain DG1 partially neutralized T cell activation induced by recombinant exoenzyme S and bound to it in an immunoblot suggesting that the epitope responsible for T cell activation is shared by exoenzyme S from strain DG1 and recombinant exoenzyme S. These data suggest that the two different preparations of exoenzyme S, despite biochemical differences, share the characteristic that is responsible for T lymphocyte activation.Key words: exoenzyme S, Pseudomonas aeruginosa, T lymphocyte, cystic fibrosis.

Pseudomonas aeruginosa exoenzyme S is a mitogen but not a superantigen for human T lymphocytes

Virtually all cystic fibrosis (CF) patients become infected with Pseudomonas aeruginosa, and once the infection is established, the organism is rarely cleared. One of the P. aeruginosa virulence factors, exoenzyme S, has been shown to correlate with increased morbidity and mortality both in rat models of chronic pulmonary inflammation and in human CF patients. It has previously been shown that exoenzyme S is a potent stimulus for the proliferation of T cells in greater than 95% of adults, which could contribute to the pathogenesis of CF. The goal of this study was to determine the mechanism of T-cell stimulation by exoenzyme S in an effort to shed light on the immune response and contribute to understanding its role in P. aeruginosa pathogenesis. The current studies demonstrate that exoenzyme S stimulates naive T cells, since fetal blood lymphocytes proliferated and adult lymphocytes that expressed CD45RA proliferated. The percentage of T cells activated by exoenzyme S after a 4-h culture (as measured by CD69 surface expression) was intermediate in magnitude compared to levels induced by a panel of superantigens and mitogens. To determine the mechanism of activation, the requirement for accessory cells was investigated. The proliferative response to exoenzyme S was dependent on the presence of accessory cells but was not blocked by an anti-DR antibody. Exoenzyme S activated both CD4(+) and CD8(+) T cells, but CD4(+) T cells were preferentially activated. The Vbeta repertoire of donor T cells showed no preferential activation or preferential expansion after stimulation by exoenzyme S, suggesting that it is not a superantigen. Taken together, our data suggest that exoenzyme S is a T-cell mitogen but not a superantigen. Activation of a large percentage of T lymphocytes by exoenzyme S may produce a lymphocyte-mediated inflammatory response that should be considered in the pathogenesis of CF.

Effects of differential expression of the 49-kilodalton exoenzyme S by Pseudomonas aeruginosa on cultured eukaryotic cells

Production of the ADP-ribosylating enzyme exoenzyme S (ExoS) by Pseudomonas aeruginosa has been associated with increased virulence. Previous studies, however, have been unable to confirm an effect of soluble ExoS in cell culture or animal model systems. To determine if bacteria must come in contact with target cells in order for an effect of ExoS to be observed, coculture systems were developed to compare the effects of ExoS- and non-ExoS-producing bacteria on eukaryotic cell function. The two P. aeruginosa strains used in these studies, 388 and 388delta exoS, maintained genetic identity, with the exception that strain 388delta exoS lacked production of the 49-kDa form of ExoS. When bacteria were cocultured with Detroit 532 fibroblastic cells, ExoS-producing 388 bacteria caused a significant decrease in DNA synthesis and viability compared to the decrease caused by non-ExoS-producing 388delta exoS bacteria. Maximal differences between the two strains were observed when 10(4) to 10(7) CFU of bacteria/ml were cocultured with Detroit cells for 4 or 6 h. Both strains were effective in eliminating Detroit cell DNA synthesis after a 20-h coculture period. Secreted ExoS had no effect on Detroit cell growth and viability, indicating that bacteria must have contact with target cells for the effect of ExoS on cellular function to be observed. Similar effects on cell proliferation and viability were observed when the two strains were cocultured with the KB epithelioid cell line. ExoS-associated decreases in eukaryotic cell viability were not found to be mediated by an inhibition of protein synthesis. These studies confirm that the 49-kDa ExoS contributes to the cellular pathogenesis of P. aeruginosa by interfering with eukaryotic cell growth and viability. In addition, the coculture system developed which recognizes this effect should provide a means for defining the function of ExoS in vivo.

Genetic analysis of exoenzyme S expression by Pseudomonas aeruginosa

The dissemination of Pseudomonas aeruginosa to the bloodstream increases the likelihood of developing fatal sepsis. In experimental models, the ability to disseminate is linked to expression of the exoenzyme S pathway. Genetic and biochemical analysis of the pathway has led to the identification of the two structural genes encoding exoenzyme S, exoS and exoT. A key regulator of several loci of the pathway has been identified as a DNA-binding protein with transcriptional activation properties. Preliminary evidence suggests that exoenzyme S and the Yop virulence determinants of yersiniae share homology among proteins involved in their synthesis and secretion. With the addition of exoS and exoT to the molecular arsenal, questions concerning in vivo toxicity and target specificities of exoenzyme S can be directly addressed.

Pseudomonas aeruginosa exoenzyme S induces proliferation of human T lymphocytes

Pseudomonas aeruginosa is a gram-negative bacterium that is responsible for devastating acute and chronic infections, which include bronchiectasis in cystic fibrosis, nosocomial pneumonia, and infection of burn wounds. Previous studies have demonstrated that these patients have impaired host responses, including cell-mediated immune responses, which are important in anti-Pseudomonas host defense. The P. aeruginosa exoproduct, exoenzyme S, has a number of characteristics which suggest that it might be important in cell-mediated immunity. To determine whether exoenzyme S activates lymphocytes to proliferate, peripheral blood mononuclear cells (PBMC) from normal volunteers were stimulated with purified exoenzyme S, and the lymphocyte response was assessed by measuring [3H]thymidine uptake and by counting the number of cells after various times in culture. Ninety-five percent of healthy adult donors had a lymphocyte response to exoenzyme S. The optimal lymphocyte response occurred on day 7, with 4 x 10(5) PBMC per microtiter well when cells were stimulated with 10 micrograms exoenzyme S per ml. [3H]thymidine uptake correlated with an increase in the number of mononuclear cells, indicating that proliferation occurred. In unseparated PBMC, T cells, and to a lesser extent B cells, proliferated. Purified T cells proliferated, while purified B cells proliferated only after the addition of irradiated T cells. Thus, T lymphocytes are necessary and sufficient for the proliferative response to exoenzyme S. We speculate that exoenzyme S from P. aeruginosa is important in T-lymphocyte-mediated host defense to P. aeruginosa. In strategies to enhance impaired cell-mediated immunity, exoenzyme S should be considered as a potential stimulant.

Construction and characterization of chromosomal insertional mutations of the Pseudomonas aeruginosa exoenzyme S trans-regulatory locus

Exoenzyme S is an ADP-ribosyltransferase produced by Pseudomonas aeruginosa. Synthesis of exoenzyme S depends on an intact trans-regulatory locus encoding three protein products, ExsC, ExsB, and ExsA. To identify the phenotype of ExsC, -B, and -A mutants in exoenzyme S production, specific insertional mutations with the streptomycin resistance-encoding omega interposon were introduced into cloned DNA and returned to the chromosomes of P. aeruginosa PA103, PAO1, and PAK. Southern blot analysis was used to confirm insertion of omega and resolution of vector sequences. Exoenzyme S expression was measured in parental and mutant derivatives by Western blot (immunoblot) analysis and ADP-ribosyltransferase activity measurement. A complete set of mutations were obtained in strains PAK and PAO1, but in strain PA103, only an insertion in the exsA coding region was identified. Southern blot analysis demonstrated that extensive duplication and rearrangement of the PA103 chromosomal trans-regulatory locus occurred when exsC::omega or exsB::omega recombination events were attempted. Exoenzyme S antigen was not detectable in the supernatant or lysate fractions of mutant strains by Western blot analysis. ADP-ribosyltransferase activity was detected in the lysate but not in the supernatant fractions of mutant derivatives. The general secretion pathway appeared to function normally in mutant strains, as elastase, exotoxin A, and phospholipase C were measured in the supernatants of parental and mutant strains. Several differences were noted when the extracellular protein profiles of parental strains were compared with similar samples from the insertional mutant strains. Some of these differences appeared to be unrelated to exoenzyme S. These data suggest that insertional inactivation of the exoenzyme S trans-regulatory locus may affect a subset of other extracellular proteins.

Helicobacter mustelae and Helicobacter pylori bind to common lipid receptors in vitro

Helicobacter pylori is a recently recognized human pathogen causing chronic-active gastritis in association with duodenal ulcers and gastric cancer. Helicobacter mustelae is a closely related bacterium with similar biochemical and morphologic characteristics. H. mustelae infection of antral and fundic mucosa in adult ferrets causes chronic gastritis. An essential virulence property of both Helicobacter species is bacterial adhesion to mucosal surfaces. The aim of this study was to determine whether H. mustelae binds to the same lipids shown previously to be receptors for H. pylori adhesion in vitro. By using thin-layer chromatography overlay and a receptor-based enzyme-linked immunosorbent assay, H. mustelae was found to bind the same receptor lipids as H. pylori, namely, phosphatidylethanolamine and gangliotetraosylceramide. In addition, both H. pylori and H. mustelae bound to a deacylplasmalogen phosphatidylethanolamine. In contrast to H. pylori, H. mustelae binding to receptors was unaffected by motility or viability. Murine monoclonal and bovine polyclonal antibodies against exoenzyme S, and exoenzyme S itself (from Pseudomonas aeruginosa), inhibited binding of H. mustelae to phosphatidylethanolamine and gangliotetraosylceramide. These findings show that H. mustelae binds in vitro to the same lipid receptors as H. pylori and suggest that the adhesion of H. mustelae to such species is mediated by preformed, surface-exposed adhesins which include an exoenzyme S-like protein.

Receptor affinity purification of a lipid-binding adhesin from Helicobacter pylori

Our previous work has shown that Helicobacter pylori specifically recognizes gangliotetraosylceramide, gangliotriaosylceramide, and phosphatidylethanolamine in vitro. This binding specificity is shared by exoenzyme S from Pseudomonas aeruginosa, and monoclonal antibodies against this adhesin prevent the attachment of H. pylori to its lipid receptors. We now report the use of a novel, versatile affinity matrix to purify a 63-kDa exoenzyme S-like adhesin from H. pylori which is responsible for the lipid-binding specificity of this organism.

Purification and characterization of exoenzyme S from Pseudomonas aeruginosa 388

Exoenzyme S was purified > 1,500-fold from the culture supernatant fluid of Pseudomonas aeruginosa 388 at high yield without utilization of solvents or detergents. Two proteins, with apparent molecular sizes of 53 and 49 kDa, cofractionated with exoenzyme S activity. Rabbit anti-49-kDa-protein immunoglobulin G was prepared by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis-purified 49-kDa protein as immunogen. Anti-49-kDa-protein IgG inhibited the ADP-ribosyltransferase activity of purified exoenzyme S in a dose-dependent manner, which indicated a role for the 49-kDa protein in the ADP-ribosylation reaction. Analysis by ultrafiltration showed that exoenzyme S activity and the 53- and 49-kDa proteins cofractionated and that exoenzyme S was apparently > 300 kDa in size. Urea (8 M) and 1.0% Triton X-100 reversibly decreased the apparent molecular sizes of exoenzyme S activity and the 53- and 49-kDa proteins to between 30 and 100 kDa.

The glycerolipid receptor for Helicobacter pylori (and exoenzyme S) is phosphatidylethanolamine

We have previously shown that Helicobacter pylori specifically binds to a glycerolipid species preferentially found in the antrum of the human stomach. We now show by high-pressure liquid chromatographic analysis that this species is a form of phosphatidylethanolamine and that H. pylori specifically binds to bona fide phosphatidylethanolamine as detected by a thin-layer chromatogram overlay procedure. Considerable variation in the binding of H. pylori to phosphatidylethanolamine from different sources was observed, however, suggesting the importance of the nature of the long-chain hydrophobic moiety. A similar binding specificity was shown by exoenzyme S from Pseudomonas aeruginosa, consistent with our hypothesis that that an exoenzyme S-like adhesin is responsible for the binding of H. pylori to its lipid receptors.

Efficient production and processing of elastase and LasA by Pseudomonas aeruginosa require zinc and calcium ions

The ability of Pseudomonas aeruginosa to degrade elastin, a major component of connective tissue, likely contributes to its pathogenicity and multiplication in human tissues. Two extracellular enzymes are required for P. aeruginosa elastolytic activity: elastase and LasA. Elastase is a zinc metalloprotease, but little is known about the structure of LasA. When grown under metal ion-deficient conditions, P. aeruginosa culture supernatants were found to exhibit a low level of elastolytic activity, which coincided with production of low levels of the 51-kDa proelastase and no detectable LasA. By using this fact to identify factors that promote elastolytic activity, P. aeruginosa PAO1, FRD2, and DG1 were grown in metal ion-deficient medium supplemented with zinc (10(-4) M ZnCl2), calcium (2.5 x 10(-3) M CaCl2), or iron (10(-4) M FeCl3). High levels of proteolytic and elastolytic activity were exhibited by all strains when cultured in the presence of both zinc and calcium, and this was associated with the production of mature 33-kDa elastase and 21-kDa LasA. Supplementing DG1 and PAO1 cultures with zinc alone stimulated the production of 33-kDa elastase, which, because of the calcium-deficient conditions, exhibited low proteolytic and elastolytic activities. Zinc also stimulated the production of a 41-kDa form of LasA in DG1 and PAO1 culture supernatants. Elastase production by FRD2 cultured in the presence of zinc alone differed from that by the other two strains in that supernatants contained 33-kDa elastase, a 21-kDa form of LasA, and exhibited high proteolytic and elastolytic activities. Such strain-associated differences in LasA processing and elastase activity can be explained by differences in metal ion-scavenging mechanisms adapted by the strains. Supplementing cultures with calcium stimulated the production of elastase but had no effect on LasA production. The elastase produced exhibited variable sizes, possibly resulting from aberrant processing reactions, and showed little proteolytic activity. Proteolytic activity could be recovered from 33-kDa elastase produced in the presence of calcium by inclusion of zinc in the enzymatic assay. Although iron was previously found to exert a repressive effect on P. aeruginosa elastolytic activity, iron exerted little effect on elastolytic activity when added to cultures containing both zinc and calcium. These studies support the conclusion that elastase production and processing are promoted by both zinc and calcium. LasA production, in comparison, is stimulated by zinc, with both zinc and calcium facilitating its processing. The association of 41-kDa LasA with a low level of elastolytic activity and of 21-kDa LasA with a high level of activity supports the conclusion that lasA encodes a larger, precursor protein which is processed to an active 21-kDa form during secretion.

Virulence determinants in Pseudomonas aeruginosa strains from urinary tract infections

A total of 121 uropathogenic Pseudomonas aeruginosa strains were examined for production of several virulence-related factors. These strains were distributed in five predominant O-serotypes, i.e. O 4, O 12, O 11, O 6 and O 5, which accounted respectively for 23.9, 23.1, 12.3, 8.2 and 5.7% of isolates. Pyochelin and pyoverdin siderophores were produced by most of the isolates, defective variants occurring at very low frequency (2.4% for pyochelin and 7.4% for pyoverdin). Adherence to uroepithelial cells and production of cytotoxins was demonstrated in 52.8 and 67.7% of the strains, respectively, with higher frequencies for epidemiologically related strains belonging to serotypes O 4 and O 12. Titration of total proteases, elastase and phospholipase C revealed a high degree of heterogeneity among isolates. However, examination of individual O-serotypes by exoenzyme production showed that elevated levels of total proteases and elastase were characteristics of serotypes of minor numerical importance, i.e. O 1, O 10, O 11 and O 17, whilst low levels of elastase were produced by strains belonging to the predominant serotypes, namely O 4 and O 12. Moreover, epidemiologically related strains belonging to serotypes O 4 and O 12 appeared more homogeneous than the whole serogroup, when compared with other groups on the basis of exoenzyme levels.

Pseudomonas aeruginosa exoenzyme S is an adhesion

Exoenzyme S from Pseudomonas aeruginosa has been studied as an adhesion for glycosphingolipids and buccal cells. Binding of exoenzyme S to gangliotriosylceramide (GalNAc beta 1-4Gal beta 1-4Glc beta 1-1Cer), gangliotetraosylceramide (Gal beta 1-3 GalNAcT beta 1-4 Gal beta 1-4Glc beta 1-1Cer), and lactosylceramide (Gal beta 1-4Glc beta 1-1Cer) separated on thin-layer chromatograms was observed. Binding curves for exoenzyme S with dilutions of gangliotetraosylceramide immobilized on plastic plates were similar to previously reported results for the intact bacteria. Binding of exoenzyme S to sialylated counterparts of these glycosphingolipids was not seen, indicating that the addition of a sialic acid residue interferes with binding. Exoenzyme S and monoclonal antibody to exoenzyme S inhibit the binding of P. aeruginosa to buccal cells. The presence of exoenzyme S on the surface of P. aeruginosa was detected by immunogold labeling of bacteria with antibodies to exoenzyme S. Results of these studies led us to conclude that exoenzyme S is an important adhesion of P. aeruginosa.

Glycolipid receptor binding specificity of exoenzyme S from Pseudomonas aeruginosa

By use of the tlc overlay procedure we have shown that exoenzyme S extracted from cultures of Pseudomonas aeruginosa specifically binds to the glycolipids asialoGM1, asialoGM2 and to a lesser extent lactosyl ceramide. More significantly, strong binding was also observed to the glycerolipid receptor we have detected for Helicobacter pylori (Lancet ii, 238-241.1989). Exoenzyme S can be extracted in a toxic and nontoxic form. Toxicity correlated with ability to bind the H. pylori receptor. This species was the only receptor detected in the most sensitive cell lines. The relative binding of exoenzyme S to the ganglio series glycolipids and the glycerolipid receptor was modified in a reciprocal manner in the presence of metal ions, suggesting that exoenzyme S has two interrelated receptor binding sites.

Cloning and expression of the Pseudomonas aeruginosa exoenzyme S toxin gene

The gene for exoenzyme S, an ADP-ribosyl transferase, was cloned from Pseudomonas aeruginosa strain DG1 using an oligonucleotide probe based on the partial N-terminal amino acid sequence to screen a library of DG1 SstI fragments inserted into pKT230 in Escherichia coli DH1. A positive clone, designated pPD3, hybridized with the oligonucleotide probe and contained a 15 kb SstI insert. In E. coli minicells pPD3 expressed a single protein of Mr 68,000. This protein was localized primarily in the periplasm in E. coli. A 3.6 kb HindIII-BamHI fragment was subcloned into the vector pT7-4 which contains the promoter from bacteriophage T7 to construct pT7-4HB. In E. coli strains expressing the T7 RNA polymerase on a second plasmid, the Mr 68,000 protein was expressed and shown to react with antibodies to exoenzyme S. No enzymatic activity was detected in cell sonicates or culture supernatants of E. coli (pPD3). Cell sonicates of E. coli (pT7-4HB) however were cytotoxic to HeLa cells and this cytotoxicity was neutralizable with anti-exoenzyme S antiserm. Thus, exoenzyme S expressed in E. coli is toxic but not enzymatically active. When plasmids carrying the exoenzyme S gene were introduced into P. aeruginosa, there was a significant increase in ADP-ribosyl transferase activity, indicating that the plasmid encoded protein is enzymatically active in P. aeruginosa.

Exoenzyme S of Pseudomonas aeruginosa ADP-ribosylates the intermediate filament protein vimentin

Exoenzyme S, which had been thought to be unselective, catalyzes the ADP-ribosylation of only a subset of cellular proteins. The intermediate filament protein vimentin is one of the more abundant substrates. Disassembled vimentin, and proteolytic fragments of vimentin that cannot form filaments, is more readily ADP-ribosylated than is filamentous vimentin.

Inhibition of Pseudomonas aeruginosa exoenzyme expression by subinhibitory antibiotic concentrations

We examined the effects of subinhibitory concentrations of ciprofloxacin, tobramycin, and ceftazidime on Pseudomonas aeruginosa exoenzyme expression in vitro and in vivo. Exotoxin A, exoenzyme S, phospholipase C, elastase, and total protease activities were suppressed by antibiotics at concentrations as low as 1/20 of the MIC over a 24-h period in broth. Continuous 10-day exposure of P. aeruginosa DG1 broth cultures to antibiotic levels equal to 1/10 of the MIC reduced exoenzyme S activity in all treatment groups. Elastase activity was reduced only by ciprofloxacin and tobramycin treatment. This suppressive effect of the antibiotics persisted throughout the 10 days and was not influenced by the increase in MIC of ciprofloxacin detected during the course of the experiment. Rats chronically infected with P. aeruginosa were treated with subinhibitory doses of antibiotics and compared with untreated controls. Bacterial numbers in lung homogenates from each of the four study groups were identical. However, the lungs from antibiotic-treated rats had significantly less histological damage than those from control rats (P less than 0.001). The protective effect was greatest for ciprofloxacin and tobramycin. Further, P. aeruginosa isolates from ciprofloxacin- and tobramycin-treated rats demonstrated significantly less exoenzyme S and elastase activity than isolates from untreated rats (P less than 0.001). Isolates from ceftazidime-treated lungs expressed less exoenzyme S activity (P less than 0.001) but an equivalent amount of elastase activity as isolates from controls. The suppression of P. aeruginosa exoenzymes may arrest progressive lung injury during chronic P. aeruginosa lung infections.