In vitro susceptibilities of 27 rickettsiae to 13 antimicrobials

The MICs of 13 antibiotics (doxycycline, thiamphenicol, rifampin, amoxicillin, gentamicin, co-trimoxazole, ciprofloxacin, pefloxacin, ofloxacin, erythromycin, josamycin, clarithromycin, and pristinamycin) were determined for 27 available rickettsial species or strains. We used two in vitro cell culture methods described previously: the plaque assay and the microplaque colorimetric assay. Our results confirm the susceptibilities of rickettsiae to doxycycline, thiamphenicol, and fluoroquinolones. Beta-lactams, aminoglycosides, and cotrimoxazole were not active. Typhus group rickettsiae were susceptible to all macrolides tested, whereas the spotted fever group rickettsiae, R. bellii, and R. canada were more resistant, with josamycin, a safe alternative for the treatment of Mediterranean spotted fever, being the most effective compound. Strain Bar 29, R. massiliae, R. montana, R. aeschlimannii, and R. rhipicephali, which are members of the same phylogenetic subgroup, were more resistant to rifampin than the other rickettsiae tested. Heterogeneity in susceptibility to rifampin, which we report for the first time, may explain in vivo discrepancies in the effectiveness of this antibiotic for the treatment of rickettsial diseases. We hypothesize that rifampin resistance and erythromycin susceptibility may reflect a divergence during the evolution of rickettsiae.     

In vitro activity of macrolides against intracellular Legionella pneumophila

The antibacterial effects of clarithromycin, azithromycin, and erythromycin were determined against five strains of Legionella pneumophila including L. pneumophila ATCC 33823 and four clinical isolates. Extracellular minimum inhibitory concentrations (MICs) and MBCs were determined by a microdilution method. Clarithromycin was the most active drug (MIC < or = 0.015-0.06), followed by azithromycin (MIC 0.03-0.12) and erythromycin (MIC 0.06-0.25). The antibacterial effect of these macrolides was then determined against L. pneumophila grown intracellularly in MRC-5 human fetal lung fibroblast cells. At two and eight times the extracellular MBC, erythromycin, azithromycin, and clarithromycin were equally effective in inhibiting growth of these five strains of intracellular L. pneumophila.     

Determination of MICs for Mycobacterium avium-M. intracellulare complex in liquid medium by a colorimetric method

We investigated the potential of a rapid colorimetric microassay based on the reduction of dimethylthiazol-diphenyltetrazolium bromide (MTT) for determining the growth of Mycobacterium avium-M. intracellulare complex (MAC) and MICs of clofazimine, resorcinomycin A, and the quinolone PD 127391 against MAC. The reduction of MTT was directly proportional to the number of viable bacteria. A comparison of the MTT reduction test with the [3H]glycerol uptake assay showed the former to possess higher analytical sensitivity for detecting MAC growth in microtiter plates. The MIT reduction test avoids the use of radioisotopes and costly material and equipment; it is reliable, reproducible, and convenient for rapid routine susceptibility testing of MAC.     

Emergence of in vitro resistance to fluoroquinolones in Neisseria gonorrhoeae isolated in Japan

To investigate emerging fluoroquinolone resistance in Neisseria gonorrhoeae isolated in Japan, we compared the in vitro antimicrobial susceptibilities of 79 gonococcal isolates from 1992 through 1993 to 14 fluoroquinolones and 14 other antibiotics with those of 27 isolates from between 1981 and 1984. The MICs at which 90% of the isolates were inhibited by nine fluroquinolones, including norfloxacin, enoxacin, ofloxacin, ciprofloxacin, tosufloxacin, lomefloxacin, fleroxacin, levofloxacin, and sparfloxacin, for isolates from 1992 to 1993 were 8- or 16-fold higher than those for isolates from 1981 to 1984. Furthermore, the MICs at which 90% of the isolates were inhibited by five fluroquinolones, including OPC-17116, T-3761, DU-6859a, AM-1155, and Q-35, that have recently been synthesized but have not yet been introduced for clinical use in Japan for isolates from 1992 to 1993 were also 2- to 16-fold higher than those for isolates from 1981 to 1984. The gonococcal isolates from 1992 to 1993 showed no significant decreases in susceptibility to beta-lactams, tetracyclines, macrolides, and spectinomycin, compared with those for isolates from 1981 to 1984. Our data indicate that the incidence of gonococcal strains with decreased susceptibilities to fluoroquinolones is increasing in Japan.     

The role of Legionella pneumophila-infected Hartmannella vermiformis as an infectious particle in a murine model of Legionnaire's disease

Legionella pneumophila is a bacterial parasite of many species of freshwater protozoa and occasionally an intracellular pathogen of humans. While protozoa are known to play a key role in the persistence of L. pneumophila in the environment, there has been limited research addressing the potential role of L. pneumophila-infected protozoa in the pathogenesis of human infection. In this report, the potential role of an L. pneumophila-infected amoeba as an infectious particle in replicative L. pneumophila lung infection was investigated in vivo with the amoeba Hartmannella vermiformis, a natural reservoir of L. pneumophila in the environment. L. pneumophila-infected H. vermiformis organisms were prepared by coculture of the amoebae and virulent L. pneumophila cells in vitro. A/J mice, which are susceptible to replicative L. pneumophila lung infection, were subsequently inoculated intratracheally with L. pneumophila-infected H. vermiformis organisms (10(6) amoebae containing 10(5) bacteria), and intrapulmonary growth of the bacteria was assessed. A/J mice inoculated intratracheally with L. pneumophila-infected H. vermiformis organisms developed replicative L. pneumophila lung infections. Furthermore, L. pneumophila-infected H. vermiformis organisms were more pathogenic than an equivalent number of bacteria or a coinoculum of L. pneumophila cells and uninfected amoebae. These results demonstrate that L. pneumophila-infected amoebae are infectious particles in replicative L. pneumophila infections in vivo and support the hypothesis that inhaled protozoa may serve as cofactors in the pathogenesis of pulmonary disease induced by inhaled respiratory pathogens.     

Use of Mono Mac 6 human monocytic cell line and J774 murine macrophage cell line in parallel antimycobacterial drug studies

The Mono Mac 6 (MM6) human monocytic cell line was evaluated with the established J774 murine macrophage cell line to ascertain its effectiveness in determining the intracellular activities of antimycobacterial drugs. Cells were infected with Mycobacterium tuberculosis H37Ra and treated with drug concentrations corresponding to the MICs, as well as to threefold higher than and threefold less than the MICs. Changes in CFU were compared after 7 days to determine significant differences between treated and nontreated groups. The results suggest that MM6 will make a useful model for testing the intracellular activities of antituberculosis drugs.     

Comparison of a photometric method with standardized methods of antifungal susceptibility testing of yeasts

We determined the fluconazole MICs for 101 clinical isolates of Candida and Cryptococcus neoformans using the macro- and microdilution methods recommended by the National Committee for Clinical Laboratory Standards. We compared the MICs obtained by these methods with those obtained by a photometric assay that quantified the reduction of 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) by viable fungi. The MIC determined by this method was defined as the highest fluconazole concentration associated with the first precipitous drop in optical density. For Candida, both the MTT and the microdilution methods demonstrated excellent agreement with the standard macrodilution method. The MTT method, however, generated MICs at 24 h that were comparable to those generated by the standard macrodilution method, whereas the microdilution method required 48 h. For C. neoformans, the levels of agreement between the MICs determined by the MTT and microdilution methods after 48 h and those determined by the standard 72-h macrodilution method were 94% (29 of 31) and 94% (29 of 31), respectively. The MTT method therefore provided results comparable to those of currently recommended methods and had the advantages of a more rapid turnaround time and potential adaptability to use as an automated system. Furthermore, the MICs determined by the MTT method were determined photometrically, thereby eliminating reader bias.     

Influence of methylprednisolone on the intracellular antimicrobial activity of erythromycin and clindamycin against Legionella pneumophila

We have investigated the effect of methylprednisolone on the intracellular activity of erythromycin and clindamycin in vitro. An assay system was developed for the determination of intracellular activity of antibiotics against Legionella pneumophila using guinea pig resident alveolar macrophages. Erythromycin at a concentration of 0.625 mg/L (5 x MIC) and clindamycin at a concentration of 8 mg/L (MIC) inhibited the growth of a single strain of L. pneumophila in macrophages, whilst ceftizoxime at a concentration of 0.625 mg/L (5 x MIC) did not. Methylprednisolone at therapeutic concentrations did not affect the intracellular antibacterial activity of either erythromycin or clindamycin against L. pneumophila. We found no direct effect of methylprednisolone on the intracellular antibacterial activity of either erythromycin or clindamycin.     

Participation of CD14 in the phagocytosis of smooth-type Salmonella typhimurium by the macrophage-like cell line, J774.1

The role of CD14 in the phagocytosis and killing of microorganisms was investigated using macrophage-like cell lines, CD14-positive J774.1 cells and CD14-negative mutant J7.DEF.3 cells derived from J744.1 cells. The cells were infected with Salmonella typhimurium organisms of the smooth (S)-form LT2, mutant rough (R)-form TV148 or Staphylococcus aureus 248betaH. At 30 or 180 min incubation, the cells were washed and disrupted. Colony-forming units (CFUs) liberated from the disrupted cells were determined by quantitative cultivation, and the phagocytic index and killing rate were calculated. Both the phagocytic index and killing rate of J774.1 cells against LT2 organisms were greater than those of J7.DEF.3 cells. However, the index and rate of J774.1 cells against TV148 and 248betaH organisms were similar to those of the J7.DEF.3 cells. The phagocytosis of LT2 organisms by J774.1 cells was partially inhibited by S-form LPS (S-LPS) and anti-CD14 antibody, but not by R-chemotype LPS (R-LPS). These results suggest that CD14 participates in the phagocytosis of S-form Salmonella.     

GH3 cells, ionic currents and cell killing: photomodification sensitized by Rose Bengal

The effects of subinhibitory concentrations (sub-MICs) of gentamicin, netilmicin and amikacin on the serum sensitivity and the cell surface hydrophobicity of Klebsiella pneumoniae were investigated. At concentrations 1/16 or 1/8 of the MICs, in all antibiotics significantly enhanced the sensitivity of K. pneumoniae to human serum as compared to nontreated bacteria. The higher concentration of the antibiotics tested was more efficient. Survival of bacteria under these conditions ranged from 24.1% to 36.7% after incubation with 10% serum for 180 min when compared with the viability of the control bacteria. Bacteria grown with the test antibiotics at sub-MICs (1/8 or 1/4 of the MICs) manifested an effective decrease of the surface hydrophobicity. The aminoglycosides which were more effective at a concentration of 1/4 of the MICs reduced bacterial hydrophobicity to 28.5% (gentamicin), 14.8% (netilmicin) and 12.7% (amikacin) of the nontreated bacteria.     

Multiple endocrine neoplasia type 1 is not rare in Japan

The ability to inhibit the in-vitro growth of mycobacteria within human monocytes is a useful screening assay for novel chemotherapeutic agents. In this study the MICs of a panel of new quinolones were determined by the broth microdilution method for two strains of Mycobacterium avium. Sixteen such compounds with MIC90s ranging from 2 to >32 mg/L were subsequently selected for the 7 day monocyte assay using ciprofloxacin for comparison. The degree of inhibition of intracellular growth correlated with the MICs. PD 139586, PD 143289, PD 135144, PD 119421 and PD 131575 were the most active new agents with activities superior to those of ciprofloxacin and sparfloxacin.     

Spontaneous intracranial hypotension: clinical and magnetic resonance imaging characteristics

OBJECTIVE: To evaluate the antimicrobial susceptibility pattern of oxacillin susceptible (OSSA) and resistant Staphylococcus aureus (ORSA) isolates to other antimicrobial agents that can be used for the treatment of staphylococcal infections. MATERIAL AND METHOD: We evaluated 117 clinical S. aureus isolates from several S?o Paulo hospitals. Clinical isolates from Campinas, SP and from Jo?o Pessoa, PB, were also included. The in vitro susceptibility testing was performed by broth microdilution as described by the National Committee for Clinical Laboratory Standards (NCCLS). The minimum inhibitory concentration (MIC) was evaluated for 24 antimicrobial agents, including beta-lactams, fluoroquinolones, aminoglycosides, glycopeptides, macrolides, lincosamides and streptogramins. Both commercially available and experimental drugs were included in the study. Cross-resistance among fluoroquinolones was evaluated by susceptibility testing 24 isolates to 10 fluoroquinolones. RESULTS: The antimicrobial agents that showed the highest in vitro activity were the glycopeptides, the streptogramin RP-59.500, and the mupirocin (100% susceptibility). Eighty-seven percent of the OSSA and only 38% of the ORSA isolates were susceptible to ciprofloxacin (MIC50 0.25 microgram/mL and > 4 micrograms/mL, respectively). Cross-resistance among fluoroquinolones were noted even for the experimental drugs. Two fluoroquinolones remained active against ciprofloxacin-resistant isolates, clinafloxacin and WIN-57.273. However, the ciprofloxacin-resistant isolates had MICs eight-to 64-fold higher than the ciprofloxacin-susceptible isolates, suggesting that the MICs may continue to increase when these fluoroquinolones become commercially available. CONCLUSION: Our results showed a high rate of antimicrobial resistance among S. aureus from the Brazilian hospitals. Very few drugs can still be used for the treatment of staphylococcal infections.     

Phosphatase-negative mutants of Legionella pneumophila and their behavior in mammalian cell infection

Microbial phosphatases are known or suspected to play a role in the pathogenesis of several intracellular pathogens, including Legionella micdadei. Legionella pneumophila also possess phosphatase activities, but their possible roles in cellular infection are unknown. We generated mutants of a serogroup 1 isolate of L. pneumophila that lack the major phosphatase. Isolation of a Pho- mutant after random mutagenesis with transposon MudII4041 allowed us to dissociate the major alkaline phosphatase (pH optimum approximately 8) from a minor acid phosphatase activity. Both activities were concentrated in the bacterial periplasm. The gene encoding the major alkaline phosphatase (pho) was cloned by expression in E. coli and used to generate a site directed mutation in two L. pneumophila strains. Each parent-mutant pair was compared in a U937 cell tissue culture assay for capacity to infect, lyse, and grow within mammalian cells. Although the parental stains differed in their U937 cell cytopathicity, neither was significantly more infective than its Pho- derivative, suggesting that the alkaline phosphatase activity is not essential for cellular infection. Because they are not attenuated, Pho- mutants can be used to generate gene fusions with E. coli alkaline phosphatase to study and secretion and cellular infectivity in L. pneumophila.     

Use of the MTT assay in adult ventricular cardiomyocytes to assess viability: effects of adenosine and potassium on cellular survival

This study used the colorimetric MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide)] assay to assess cell viability in isolated quiescent adult guinea-pig ventricular myocytes exposed to different insults or cardioprotective conditions, including adenosine and hyperkalemic-cardioplegia. Optical density (OD), reflecting intracellular reduction of MTT into formazan pigment formation, was a function of the number of viable cells (coefficient of linear correlation approximately 0.99), with MTT reduction preferentially carried out by rod-shaped cardiomyocytes (absorbance at 1.009 +/- 0.013 and 0.006 +/- 0.001 OD units for populations containing 50 and 0% of rod-shaped cells). Following prolonged mechanical (pressure of 1 lb/min for 40 min), chemical (10% DMSO or ethanol) or hypoxic injury (N2-saturated solution), the MTT reductase activity reflected reduction in the number of viable cells by 87%, >50%, and 77%, respectively. In cardiomyocytes exposed to a 40 min hypoxia (with CO2), the MTT reductase activity was 0.056 +/- 0.009 in the absence, and 0.074 +/- 0.008 OD units in the presence of adenosine (1 mM), i.e. adenosine reduced the number of non-viable cells. Also, the MTT assay revealed that the effect of potassium-containing solutions (16 and 32 mM K+) on cellular viability may depend on the extent of insult imposed on cardiomyocytes; i.e. a approximately 24% and 49% increase under mild hypoxia (0.03% CO2), or an 18% decrease in cell viability under severe hypoxia (N2) in pre-injured cells. Thus, the MTT assay used to assess viability of isolated adult cardiomyocytes revealed a direct cytoprotective effect of adenosine and hyperkalemic-cardioplegia by promoting cell survival under certain conditions in vitro.     

Differential chemosensitivity in oncogene-transformed cells

The effects of anticancer drugs on cell growth have been investigated by MTT colorimetric assay using mouse and rat cultured cells transformed by various oncogenes and tumor viruses. Aclarubicin, mitomycin C and 1-hexylcarbamoyl-5-fluorouracil showed higher growth-inhibitory activities toward transformed cells than those toward the normal counterparts. Nimustine, bleomycin and 5-fluorouracil also showed selective growth-suppressive activities toward transformed cells except for a few cell lines. ras-oncogene-transformed cells were more sensitive toward 5-fluorouracil and 1-hexylcarbamoyl-5-fluorouracil than the normal parent cells and other transformed cells. These drugs would thus be effective in the chemotherapy of ras-induced cancer.     

In vitro activities of the oxazolidinone antibiotics U-100592 and U-100766 against Staphylococcus aureus and coagulase-negative Staphylococcus species

U-100592 and U-100766 are closely related antibiotics of the oxazolidinone class. Their in vitro activities were determined against 100 isolates of Staphylococcus aureus and 100 isolates of coagulase-negative Staphylococcus species by broth and agar dilution test methods. The MICs of both compounds by either test method at which 50 and 90% of isolates are inhibited were 2 and 4 micrograms/ml, respectively, for S. aureus and 1 to 2 micrograms/ml for coagulase-negative staphylococci. Time-kill assay with selected strains indicated a primarily bacteriostatic effect against staphylococci.     

Surface-associated hsp60 chaperonin of Legionella pneumophila mediates invasion in a HeLa cell model

HeLa cells have been previously used to demonstrate that virulent strains of Legionella pneumophila (but not salt-tolerant avirulent strains) efficiently invade nonphagocytic cells. Hsp60, a member of the GroEL family of chaperonins, is displayed on the surface of virulent L. pneumophila (R. A. Gardu?o et al., J. Bacteriol. 180:505-513, 1988). Because Hsp60 is largely involved in protein-protein interactions, we investigated its role in adherence-invasion in the HeLa cell model. Hsp60-specific antibodies inhibited the adherence and invasiveness of two virulent L. pneumophila strains in a dose-dependent manner but had no effect on the association of their salt-tolerant avirulent derivatives with HeLa cells. A monospecific anti-OmpS (major outer membrane protein) serum inhibited the association of both virulent and avirulent strains of L. pneumophila to HeLa cells, suggesting that while both Hsp60 and OmpS may mediate bacterial association to HeLa cells, only virulent strains selectively displayed Hsp60 on their surfaces. Furthermore, the surface-associated Hsp60 of virulent bacterial cells was susceptible to the action of trypsin, which rendered the bacteria noninvasive. Additionally, pretreatment of HeLa cells with purified Hsp60 or precoating of the plastic surface where HeLa cells attached with Hsp60 reduced the adherence and invasiveness of the two virulent strains. Finally, recombinant Hsp60 covalently bound to latex beads promoted the early association of beads with HeLa cells by a factor of 20 over bovine serum albumin (BSA)-coated beads and competed with virulent strains for association with HeLa cells. Hsp60-coated beads were internalized in large numbers by HeLa cells and remained in tight endosomes that did not fuse with other vesicles, whereas internalized BSA-coated beads, for which endocytic trafficking is well established, resided in more loose or elongated endosomes. Mature intracellular forms of L. pneumophila, which were up to 100-fold more efficient than agar-grown bacteria at associating with HeLa cells, were enriched for Hsp60 on the bacterial surface, as determined by immunolocalization techniques. Collectively, these results establish a role for surface-exposed Hsp60 in invasion of HeLa cells by L. pneumophila.     

Temperature effects on Legionella pneumophila killing by and multiplication in phagocytes of guinea pigs

We examined the effects of temperature on the interaction between Legionella pneumophila and phagocytes of guinea pigs. The body temperatures of guinea pigs infected with a sublethal dose (1.2 x 10(4) CFU) or a lethal dose (1.0 x 10(5) CFU) of L. pneumophila elevated from 38.4 +/- 0.15 C to 40.2 +/- 0.42 C or 40.3 +/- 0.62 C, respectively. The intracellular bacterial killing by and bacterial proliferation in the phagocytes were examined at 33, 37, 40, and 42 C, using in vitro culture systems of peritoneal macrophages or polymorphonuclear leukocytes (PMN) of guinea pigs. In all the macrophages incubated at different temperatures, significant intracellular bacterial killings were observed at 4 hr after in vitro phagocytosis. After 24 hr of incubation, there was about a 100-fold increase of CFU and the number reached a maximum after 48 hr of incubation in the macrophages incubated at 42 C as well as 37 and 40 C, suggesting that macrophages support the intracellular bacterial growth in hyperthermia. In the PMN, L. pneumophila CFU 4 hr or 12 hr after the infection were significantly lower at 42 C than those at 37 C (P < 0.05), indicating that the bactericidal capacity of PMN was enhanced at 42 C compared to 37 C. However, in all the PMN incubated at different temperatures, there were about 10-fold increases of CFU 24 hr after the infection, suggesting that PMN as well as macrophages support intracellular bacterial growth in hyperthermia. The extracellular bacterial growth was examined at 33, 37, 40, and 42 C in buffered yeast extract (BYE) broth or RPMI 1640 medium containing 50% guinea pig serum as a permissive or non-permissive liquid medium for the bacterial growth, respectively. Inhibition of bacterial growth in BYE broth at 42 C, and a decrease of CFU in RPMI 1640 medium containing 50% guinea pig serum at 42 C were observed. In conclusion, hyperthermia may be beneficial by restricting extracellular bacterial survival, but it exerts no beneficial effect on the restriction of intracellular bacterial growth in phagocytes, though PMN showed enhanced initial killing at 42 C. These results suggest that fever, or hyperthermia itself, may not largely contribute as a nonspecific host defense early in the course of legionellosis.     

Photosensitization of skin-derived cell lines by Dimegin [2,4-di-(alpha-methoxyethyl)-deuteroporphyrin IX] in vitro

The deuteroporphyrin-IX derivative Dimegin [2,4-di-(alpha-methoxyethyl)-deuteroporphyrin IX] was investigated with respect to cellular uptake, intracellular localization and cell survival following photodynamic treatment in human cell lines derived from the skin (SCL1 and SCL2, squamous cell carcinoma; HaCaT keratinocytes; N1 fibroblasts). Using flow cytometry, we determined the cellular fluorescence as a marker of the uptake of Dimegin after incubation for 24 h. The intracellular localization of Dimegin was analysed using fluorescence microscopy and co-staining with fluorescent dyes specific for cell organelles. Following irradiation with an incoherent light source (580-740 nm) using a light dose of 24 J/cm2, phototoxicity was determined by means of trypan blue dye exclusion, MTT assays and growth curves. The relative Dimegin fluorescence of the different cell lines declined as follows: SCL1 > HaCaT > N1 > SCL2. Intracellular localization of Dimegin was found in the mitochondria. For all cell lines Dimegin concentrations above 15 microM yielded a significant phototoxic effect. The EC50 for SCL1 cells was 8.9 +/- 2.0 microM Dimegin. The EC50 for the cell lines increased as follows: SCL1 < HaCaT < N1 < SCL2, thus correlating with the cellular fluorescence of Dimegin. The results of the MTT assay were confirmed by trypan blue dye exclusion assay and growth curves. In conclusion, the study shows that Dimegin is an effective photosensitizer with a rapid mechanism of action in vitro, resulting in an immediate loss of plasma membrane integrity following irradiation.     

Humoral immunity and regulation of intrapulmonary growth of Legionella pneumophila in the immunocompetent host

The potential role of humoral immunity in regulating intrapulmonary growth of Legionella pneumophila in the immunocompetent host was investigated using a murine model of Legionnaires' disease. Intratracheal inoculation of A/J mice with a virulent strain of L. pneumophila (10(6) bacteria per mouse) resulted in the recruitment of B lymphocytes into the lung and the development of anti-L. pneumophila Ab. Opsonization of L. pneumophila in vitro with anti-L. pneumophila-specific mAb resulted in a significant decrease in intrapulmonary growth of the bacteria at 24 to 72 h postinfection. Transmission electron microscopic analysis of lung tissue from L. pneumophila- infected mice demonstrated that while there was no significant difference between phagocytosis of the unopsonized and opsonized L. pneumophila by alveolar macrophages at 24 h postinfection, phagocytosis of opsonized bacteria by alveolar mononuclear phagocytic cells was significantly enhanced at 48 h postinfection. Depletion of A/J mice of complement before intratracheal inoculation of opsonized L. pneumophila (10(6) bacteria per mouse) did not significantly alter intrapulmonary growth of L. pneumophila. These results suggest that anti-L. pneumophila Ab, produced during replicative L. pneumophila lung infections, may regulate intrapulmonary growth of L. pneumophila in the immunocompetent host by decreasing the viability of extracellular L. pneumophila and by enhancing phagocytosis of the bacteria by alveolar mononuclear phagocytic cells by a complement-independent mechanism.