Quantitative and cost comparison of ultrasensitive human immunodeficiency virus type 1 RNA viral load assays: Bayer bDNA quantiplex versions 3.0 and 2.0 and Roche PCR Amplicor monitor version 1.5.

Quantification of human immunodeficiency virus type 1 (HIV-1) RNA as a measure of viral load has greatly improved the monitoring of therapies for infected individuals. With the significant reductions in viral load now observed in individuals treated with highly active anti-retroviral therapy (HAART), viral load assays have been adapted to achieve greater sensitivity. Two commercially available ultrasensitive assays, the Bayer Quantiplex HIV-1 bDNA version 3.0 (bDNA 3.0) assay and the Roche Amplicor HIV-1 Monitor Ultrasensitive version 1.5 (Amplicor 1.5) assay, are now being used to monitor HIV-1-infected individuals. Both of these ultrasensitive assays have a reported lower limit of 50 HIV-1 RNA copies/ml and were developed from corresponding older generation assays with lower limits of 400 to 500 copies/ml. However, the comparability of viral load data generated by these ultrasensitive assays and the relative costs of labor, disposables, and biohazardous wastes were not determined in most cases. In this study, we used matched clinical plasma samples to compare the quantification of the newer bDNA 3.0 assay with that of the older bDNA 2.0 assay and to compare the quantification and costs of the bDNA 3.0 assay and the Amplicor 1.5 assay. We found that quantification by the bDNA 3.0 assay was approximately twofold higher than that by the bDNA 2.0 assay and was highly correlated to that by the Amplicor 1.5 assay. Moreover, cost analysis based on labor, disposables, and biohazardous wastes showed significant savings with the bDNA 3.0 assay as compared to the costs of the Amplicor 1.5 assay.

Clinical utility of a commercial test based on the polymerase chain reaction for detecting Mycobacterium tuberculosis in respiratory specimens.

Several studies have reported using methods based on polymerase chain reaction (PCR) to detect Mycobacterium tuberculosis in respiratory tract specimens. However, little is known about the actual clinical utility of PCR-based tests, and it is uncertain if PCR technology can be transferred to the clinical laboratory. To determine its utility, we evaluated a commercially developed PCR test system in a clinical laboratory using consecutive respiratory tract specimens. Microscopic examination of smears stained with acid-fast bacilli (AFB), culture, and a PCR-based test (Amplicor Mycobacterium tuberculosis assay; Roche Molecular Systems) were used to evaluate 535 consecutive sputum and bronchoalveolar lavage specimens from 227 patients. A clinical case definition of tuberculosis was used as the reference-standard to determine the utility of all diagnostic tests. For all specimens from patients with a new or a treatment-failure case of pulmonary tuberculosis, the positivity rate of PCR (58%) was similar to that of culture (56%) (p > 0.90) and substantially greater than microscopic examination of AFB-stained smears (22%) (p < 0.001). PCR and culture detected M. tuberculosis in 46 and 43%, respectively, of the specimens from patients who did not have AFB on microscopic examination of their respiratory tract specimens (p > 0.90). PCR had a false positive rate of 0.8%. In several instances, PCR detected M. tuberculosis when culture did not; and vice versa. The clinical utility of this PCR-based test is similar to that of culture for detecting M. tuberculosis in respiratory tract specimens.(ABSTRACT TRUNCATED AT 250 WORDS)

Mycobacterium avium complex in water, food, and soil samples collected from the environment of HIV-infected individuals.

As part of an epidemiologic study of Mycobacterium avium complex (MAC) infection in San Francisco, water, food and soil samples were collected from the home environment of 290 persons with human immunodeficiency virus (HIV) infection and cultured for mycobacteria. Isolates recovered from the environment were compared with isolates cultured from study patients. Although mycobacteria were recovered from numerous environmental samples, isolates reactive with MAC-specific DNA probes were recovered from only four of 528 (0.76%) water samples and one of 397 (0.25%) food samples. The species M. avium was recovered from one water (0.19%) and one food sample. In contrast, MAC was recovered from 55% and M. avium from 27% of soil samples taken from potted plants in patients' home. Speciation of 76 MAC isolates from study patients showed all isolates belonged to the species M. avium. With use of serotype and multilocus enzyme electrophoresis analysis, some of the soil isolates were found to be similar to isolates recovered from study patients. The results of this study suggest that soil, rather than water, may be a significant reservoir of organisms causing MAC infection in San Francisco.

High predictive value of the acid-fast smear for Mycobacterium tuberculosis despite the high prevalence of Mycobacterium avium complex in respiratory specimens.

The value of the smear for acid-fast bacilli in predicting pulmonary tuberculosis is unclear in a setting where there is a high prevalence of Mycobacterium avium complex in respiratory specimens. To evaluate the impact of a high prevalence of M. avium complex on the predictive value of the acid-fast bacilli smear for tuberculosis, we reviewed findings on smears and results of cultures over a 3-year period at a hospital where M. avium complex is the predominant mycobacterial isolate. In this setting, the predictive value of the acid-fast bacilli smear for Mycobacterium tuberculosis was 92% for expectorated sputum specimens, 71% for induced sputum specimens, and 71% for bronchoalveolar lavage specimens. When multiple specimens collected from the same patient were excluded from the data base, the predictive values were 87%, 70%, and 71%, respectively. Smears of sputum samples were positive at the same rate for patients with tuberculosis who had AIDS and for patients with tuberculosis who did not have AIDS.

Environmental risk factors for acquisition of Mycobacterium avium complex in persons with human immunodeficiency virus infection.

A case-control study was done to determine risk factors for Mycobacterium avium complex (MAC) disease in persons infected with human immunodeficiency virus (HIV) with < 50 CD4+ cells/mm3. In univariate analysis, cases (n = 83) had lower CD4+ cell counts than controls (n = 177) (median, 10 vs. 17/mm3; P < .001) and were more likely to have consumed hard cheese (odds ratio [OR], 5.44; 95% confidence interval [CI], 1.61-18.4) but were less likely to have taken daily showers (OR, 0.55; 95% CI, 0.33-0.94). In multivariate analysis, CD4+ cell count < 25/mm3 (OR, 3.58; 95% CI, 1.71-7.49) and consumption of hard cheese (OR, 5.63; 95% CI, 1.58-20.1) remained associated with disease, while daily showering (OR, 0.58; 95% CI, 0.28-0.88) remained protective. Increased risk for MAC disease in persons with HIV infection and low CD4+ cell counts is not associated with exposure to water or a variety of other environmental sources but may be associated with consumption of hard cheese.

Mycobacterium avium complex in the respiratory or gastrointestinal tract and the risk of M. avium complex bacteremia in patients with human immunodeficiency virus infection.

Mycobacterium avium complex (MAC) is frequently isolated from the respiratory or gastrointestinal tract of patients with advanced human immunodeficiency virus (HIV) infection. Whether they are at increased risk of MAC bacteremia and whether culture of respiratory tract or stool specimens is useful for predicting bacteremia are unclear. HIV-infected patients with < or = 50 CD4+ cells/microL were prospectively studied. The risk of MAC bacteremia was approximately 60% within 1 year for patients with MAC in either the respiratory or gastrointestinal tract and was greater than for those without MAC in these sites (relative hazards for respiratory and gastrointestinal tract, 2.3 and 6.0; 95% confidence intervals, 1.1-4.6 and 2.5-14.6, respectively). Both respiratory tract specimen and stool culture had poor sensitivities (22% and 20%, respectively) but good positive predictive values (approximately 60%) for bacteremia. Symptomatic HIV-infected patients with MAC in the respiratory or gastrointestinal tract are at a substantial risk for developing MAC bacteremia; culture of these sites has limited usefulness as a screening test.

Comparison of four decontamination methods for recovery of Mycobacterium avium complex from stools.

The presence of Mycobacterium avium complex (MAC) in stool specimens may be a predictor of disseminated MAC infection, yet the methods for decontaminating stools have not been evaluated for their usefulness in recovering MAC organisms. In the present study, four decontamination methods commonly used to recover acid-fast bacteria from respiratory specimens were compared for their utility in recovering MAC from stool specimens. Ten strains of MAC were used at a level of 10(4) to 10(6) CFU to seed the stool specimens. Specimens were divided into four portions and were decontaminated by using the following treatments: (i) N-acetyl-L-cysteine-sodium hydroxide (NALC-NaOH), (ii) cetylpyridinium chloride-sodium chloride (CPC-NaCl), (iii) oxalic acid, or (iv) benzalkonium chloride-trisodium phosphate (BC-TSP). The specimens were then plated onto a total of five pieces of selective and nonselective egg- and agar-based media. The oxalic acid method yielded the greatest number of MAC CFU from seeded stool samples; this was followed by NALC-NaOH, BC-TSP, and CPC-NaCl. The difference between the oxalic acid method and each of the other methods was statistically significant (analysis of variance at the 95% significance level). Although more MAC CFU was recovered from seeded stool samples by using oxalic acid than NALC-NaOH, no difference in culture positivity rates was observed when the two methods were used to test 368 clinical stool specimens processed with either oxalic acid (164 specimens) or NALC-NaOH (204 specimens) (P = 0.07) or 67 specimens processed by both methods (P = 0.77). The oxalic acid and NALC-NaOH decontamination methods both appear to be useful for the recovery of MAC organisms from stool specimens.

Efficacies of liposome-encapsulated streptomycin and ciprofloxacin against Mycobacterium avium-M. intracellulare complex infections in human peripheral blood monocyte/macrophages.

Current treatments of disseminated infection caused by the Mycobacterium avium-M. intracellulare complex (MAC) are generally ineffective. Liposome-mediated delivery of antibiotics to MAC-infected tissues in vivo can enhance the efficacy of the drugs (N. Düzgünes, V. K. Perumal, L. Kesavalu, J. A. Goldstein, R. J. Debs, and P. R. J. Gangadharam, Antimicrob. Agents Chemother. 32:1404-1411, 1988; N. Düzgünes, D. A. Ashtekar, D. L. Flasher, N. Ghori, R. J. Debs, D. S. Friend, and P. R. J. Gangadharam, J. Infect. Dis. 164:143-151, 1991). We investigated the therapeutic efficacies of liposome-encapsulated streptomycin and ciprofloxacin against growth of the MAC inside human peripheral blood monocyte/macrophages. Treatment was initiated 24 h after infection of macrophages with the MAC and stopped after 20 h, and the cells were incubated for another 7 days. The antimycobacterial activity of streptomycin was enhanced when the drug was delivered to macrophages in liposome-encapsulated form, reducing the CFU about threefold more than the free drug did throughout the concentration range studied (10 to 50 micrograms/ml). With 50 micrograms of encapsulated streptomycin per ml, the CFU were reduced to 11% of the initial level of infection. Liposome-encapsulated ciprofloxacin was at least 50 times more effective against the intracellular bacteria than was the free drug: at a concentration of 0.1 microgram/ml, liposome-encapsulated ciprofloxacin had greater antimycobacterial activity than the free drug at 5 microgram/ml. With liposome-encapsulated ciprofloxacin at 5 micrograms/ml, the CFU were reduced by more than 1,000-fold at the end of the 7-day incubation period, compared with untreated controls. These results suggest that liposome-encapsulated ciprofloxacin or other fluoroquinolones may be effective against MAC infections in vivo.

Use of DNA probes to detect Mycobacterium intracellulare and "X" mycobacteria among clinical isolates of Mycobacterium avium complex.

A mycobacterial DNA probe (designated X) was recently developed to help identify Mycobacterium avium complex (MAC) isolates that are nonreactive with probes specific for M. avium or Mycobacterium intracellulare. The prevalence of X probe-positive mycobacteria in clinical specimens and their role in causing disease is unknown. Using a DNA probe kit that includes the X probe, we characterized 100 consecutive clinical MAC isolates as M. avium, M. intracellulare, or X. Lysates from 81 of the isolates reacted with the M. avium probe, 13 with the M. intracellulare probe, 3 with the X probe, and 3 failed to hybridize with any of the probes. All three X-positive isolates were recovered from sputa of patients who were recent immigrants to the United States and who presented with hemoptysis. One isolate was from a Hispanic man infected with human immunodeficiency virus type 1 (HIV-1) and the other 2 were from Filipino patients with no HIV-1 risk factors. This study also showed a higher than expected number of M. intracellulare isolates from blood and cerebrospinal fluid of HIV-1-infected patients.

Comparison of the intracellular activities of clarithromycin and erythromycin against Mycobacterium avium complex strains in J774 cells and in alveolar macrophages from human immunodeficiency virus type 1-infected individuals.

The intracellular activities of clarithromycin and erythromycin, alone and in combination with other antimicrobial agents, were tested against Mycobacterium avium complex (MAC) strains inside mouse J774 cells and inside alveolar macrophages obtained from human immunodeficiency type 1-infected individuals. Clarithromycin alone had greater intracellular activity than erythromycin alone, and drug combinations that included clarithromycin were usually more active than combinations that included erythromycin.

Effects of antimicrobial agents on survival of Mycobacterium avium complex inside alveolar macrophages obtained from patients with human immunodeficiency virus infection.

Measurements of the activities of antimicrobial agents against the Mycobacterium avium complex (MAC) usually do not take into consideration the intracellular location of the organism. A recent study using mouse macrophage continuous cell line J774 (D. M. Yajko, P.S. Nassos, C. A. Sanders, and W. K. Hadley, Am. Rev. Respir. Dis. 140: 1198-1203, 1989) showed that certain combinations of antimicrobial agents are able to kill MAC inside macrophages and suggested that the J774 cell line could be used as a model for screening of drugs for intracellular activity against MAC. As a test of the validity of this model, alveolar macrophages were isolated from the bronchoalveolar lavages of 36 patients who had AIDS or an AIDS-related condition or were considered to be at risk for AIDS. The macrophages were infected with MAC and then treated with a drug or drug combination for 48 to 72 h. Survival of MAC was measured over time in drug-treated macrophages and untreated control macrophages. No single drug or two-drug combination that was tested was able to cause a decrease in the survival of every one of the MAC strains used in the study. However, several three-drug combinations that had been shown to cause a decrease in survival of all MAC strains inside J774 cells also caused a decrease in survival of all MAC strains inside alveolar macrophages from patients. The good agreement between these results and those obtained previously with J774 cells gives further evidence of the usefulness of the simpler J774 model for screening of drugs for intracellular activity against MAC.

Prevalence of Mycobacterium avium complex in respiratory specimens from AIDS and non-AIDS patients in a San Francisco hospital.

Over the past several years there has been a large increase in the recovery of Mycobacterium avium complex (MAC) isolates from respiratory specimens submitted to the clinical laboratory at San Francisco General Hospital (SFGH). This increase in MAC recovery correlates with an increase in the number of cases of acquired immunodeficiency syndrome (AIDS) in the community. Although it is well known that MAC is often isolated from patients with AIDS, the isolation of MAC from respiratory specimens is often attributed to contamination of the specimen with MAC organisms present in the environment. To determine whether the increase in MAC isolates recovered at SFGH was due to an increase in environmental contamination of specimens or to the increase in our AIDS patient population, we conducted a study of the prevalence of MAC in respiratory specimens from AIDS versus non-AIDS patients. Results of specimens submitted to the clinical laboratory at SFGH for culture of mycobacteria were reviewed over a 12-yr period, from 1977 through 1988. The prevalence of MAC in respiratory specimens from AIDS and non-AIDS patients was determined for 4 yr during this period: the pre-AIDS year 1977; the first year AIDS was reported in San Francisco, 1981; 1984; and 1987. In 1977 and 1981 the prevalence of MAC in respiratory specimens was less than or equal to 0.5%, and all MAC isolates were recovered from non-AIDS patients. In 1984 the prevalence of MAC in respiratory specimens for AIDS and non-AIDS patients was 6.5 and 0.3%, respectively, and in 1987, 8.8 and 0.3%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Detection and analysis by dual-laser flow cytometry of bacteriophage T4 DNA inside Escherichia coli.

Bacteriophage T4 DNA was detected and analyzed inside E. coli by dual-laser flow cytometry using a dye combination of Hoechst 33258 (H33258) and chromomycin A3 (CA3) which bind to A-T- and G-C-rich regions of DNA, respectively. An exponentially-growing culture of E. coli ATCC 11303 was infected with T4 bacteriophage at a 1:1 multiplicity of infection. Samples were taken immediately and at 5 min intervals and placed on ice to interrupt viral replication. The samples were then centrifuged, ethanol-fixed, stained with H33258 and CA3, and analyzed by flow cytometry. Twenty-five minutes post-infection, a population of cells which contained T4 DNA began to appear on both a bivariate contour plot and a frequency histogram plot of the data. By 35 min, T4 DNA-containing cells could be distinguished from E. coli cells containing little or no T4 DNA. The ratio of CA3:H33258 fluorescence was then used to calculate the % G + C value for T4 DNA inside E. coli. A value of 35.6 +/- 0.2% was obtained, which agrees with % G + C values determined by traditional methods. These results demonstrate that dual-laser flow cytometry can be used to study viral DNA inside the bacterial host.

In vitro susceptibility of Mycobacterium avium complex to the new fluoroquinolone sparfloxacin (CI-978; AT-4140) and comparison with ciprofloxacin.

We tested the activity of the new fluoroquinolone sparfloxacin (CI-978; AT 4140) against 30 strains of Mycobacterium avium complex (MAC) isolated from patients with acquired immune deficiency syndrome. MICs of sparfloxacin (range, less than or equal to 0.06 to 4 micrograms/ml) were lower than MICs of ciprofloxacin for all 30 strains, and MBCs for acid-fast bacteria were lower for 28 of the 30 strains. In synergism experiments using 10 strains of MAC, fractional inhibitory concentration indices revealed that the combination of sparfloxacin plus ethambutol was synergistic against 9 strains, and the three-drug combination of sparfloxacin plus ethambutol plus rifampin was synergistic against all strains. In the absence of ethambutol, the combination of sparfloxacin plus rifampin appeared to be antagonistic against three of the MAC strains.

Evaluation of an indirect fluorescent-antibody stain for detection of Pneumocystis carinii in respiratory specimens.

Two prospective studies were undertaken to evaluate a commercial indirect fluorescent-antibody (IFA) stain for the detection of Pneumocystis carinii in respiratory specimens from individuals at risk for or with the acquired immunodeficiency syndrome. The first study compared IFA with Diff-Quik (DQ; a rapid Giemsa-like stain) for detecting P. carinii in 95 induced sputa obtained from 77 asymptomatic patients who had survived one previous episode of P. carinii pneumonia and who were being treated prophylactically with aerosolized pentamidine. Only one induced sputum specimen was found to contain P. carinii; organisms were detected by both stains. The second study compared the performance of the IFA stain versus DQ, modified toluidine blue O, and Gomori methenamine silver stains for detecting P. carinii in symptomatic individuals at risk for or with acquired immunodeficiency syndrome. Of 182 specimens examined, P. carinii was detected in 105 by one or more stains; the DQ stain detected 73 (70%), the modified toluidine blue O stain detected 75 (71%), the Gomori methenamine silver stain detected 76 (72%), and the IFA stain detected 95 (90%). The IFA stain was more sensitive (P less than 0.01) than the other traditional stains for detecting P. carinii; however, a subsequent clinical evaluation revealed that a subset of IFA-positive-only specimens were from patients whose clinical symptoms resolved without specific anti-P. carinii therapy.

Determination of guanine-plus-cytosine content of bacterial DNA by dual-laser flow cytometry.

A dual-laser flow cytometer was used to analyse different species of bacteria for the molar percentage of guanine-plus-cytosine (% G + C) without the need for DNA extraction or purification. Ethanol-fixed bacterial cells were stained with a combination of DNA-specific fluorochromes, Hoechst 33258 and chromomycin A3, which bind to AT- and GC-rich regions of DNA, respectively. A linear relationship (r = 0.99) was demonstrated between the log of the ratio of chromomycin A3 to Hoechst 33258 fluorescence and the log of the % G + C as determined by thermal denaturation (Tm) or buoyant density centrifugation (Bd) methods. Linearity was maintained for all bacterial species tested over the range of 28-67% G + C. A standard curve was constructed using five strains whose % G + C had been determined by other methods. From the equation describing this line, the % G + C values of nine other strains with known DNA base composition, together with the five strains used to construct the curve, were calculated using the chromomycin A3 to Hoechst 33258 ratio and were in agreement with values obtained by Tm, Bd or HPLC. The reproducibility of flow cytometric analysis (mean error 0.7% G + C) compared well with the reproducibility of other methods. Mixtures containing two species were also analysed. Two cell populations could be discerned in mixtures containing two species which differed in base composition by as little as 4% G + C.(ABSTRACT TRUNCATED AT 250 WORDS)

Killing by antimycobacterial agents of AIDS-derived strains of Mycobacterium avium complex inside cells of the mouse macrophage cell line J774.

The murine macrophage continuous cell line J774 was used to measure the ability of antimicrobial agents, either singly or in combination, to kill intracellular Mycobacterium avium complex. All of 14 strains of M. avium complex, isolated from patients with AIDS, grew inside J774 cells during an incubation period of 7 days. The susceptibility of macrophage-ingested M. avium complex to antimicrobial agents was determined by comparing the number of colony-forming units (cfu) of M. avium complex inside untreated macrophages at the time of drug addition with the number of cfu present in macrophages after treatment with drugs for 7 days. Simultaneous experiments were carried out in broth medium without macrophages in order to compare killing of free mycobacteria with killing of macrophage-ingested mycobacteria. Antimicrobial agents (rifampin, rifabutin [Ansamycin], ethambutol, ciprofloxacin, clofazimine, isoniazid, and amikacin) were tested using concentrations that are achievable in the serum of patients. Among drugs known to penetrate macrophages, there was 96.2% agreement in susceptibility test results between the broth experiments and the J774 experiments when single drugs were tested, but only 74% agreement when combinations of drugs were tested. Killing of M. avium complex inside J774 cells by any single drug was uncommon. However, killing in J774 cells occurred against 10 of 11 (91%) strains with the combination of rifabutin + ethambutol + ciprofloxacin and against all of seven strains tested with the combination of rifabutin + ethambutol + amikacin. Interpretive criteria of in vitro susceptibility data need to be developed so that these interpretations correlate with a predictable clinical response in patients.

Antimicrobial synergism against Mycobacterium avium complex strains isolated from patients with acquired immune deficiency syndrome.

Pairs of 11 antimicrobial agents were tested in vitro for their ability to act synergistically against three strains of Mycobacterium avium complex isolated from patients with acquired immune deficiency syndrome. From the combinations tested, four drugs (ethambutol, rifampin, ciprofloxacin, and erythromycin) were selected for more extensive study against 20 strains of M. avium complex. The inhibitory and killing synergism obtained with combinations of two, three, or four drugs was assessed by determining the fractional inhibitory concentration index and fractional bactericidal concentration index. Inhibitory synergism occurred against 90 to 100% of the strains for all drug combinations in which ethambutol was included. Killing synergism occurred against 85 to 95% of the strains when ethambutol was used in combinations which included either rifampin or ciprofloxacin. However, killing synergism occurred against only 45% of the strains when drugs were tested at concentrations that can be obtained in patient serum. In other experiments, rifabutin (Ansamycin) gave results that were comparable to those obtained with rifampin. Clofazimine did not show synergistic killing activity at a concentration that is achievable in serum for any of the drugs tested. Our results indicate that there is considerable variability in the antimicrobial susceptibility of M. avium isolates obtained from patients with acquired immune deficiency syndrome. This variability could have significant impact on the clinical response to various therapies.

Lactic acid concentration as an indicator of acceptability in refrigerated or freeze-thawed ground beef.

Lactic acid concentrations increased in refrigerated and freeze-thawed anaerobically stored ground beef. Bacterial counts were higher in refrigerated samples, but the ratios of gram-positive bacteria in refrigerated and freeze-thawed samples were the same. No differences in appearance or odor between refrigerated and freeze-thawed samples were noted after 2 days of aerobic storage. Initial lactic acid concentration can be used to predict the shelf life of frozen beef.

Broth microdilution testing of susceptibilities to 30 antimicrobial agents of Mycobacterium avium strains from patients with acquired immune deficiency syndrome.

A total of 31 strains of Mycobacterium avium complex isolated from patients with acquired immune deficiency syndrome were tested for susceptibility to 30 antimicrobial agents by using microdilution trays containing dried antimicrobial agents. MICs were determined over a period of 7 days of growth in a broth medium (7HSF) that is equivalent to 7H11 agar. MICs obtained by this method showed good agreement with MICs determined by the agar dilution method. Strains could be divided into two groups by their antimicrobial susceptibility patterns. All group 1 strains (8 of the 31 strains tested) were at least moderately susceptible to inhibition by a variety of beta-lactam antimicrobial agents, including amoxicillin-clavulanic acid and cefmenoxime. Group 2 strains (23 of 31) were susceptible only to amikacin (22 of 23 strains). All 31 strains were resistant to oxacillin, clindamycin, erythromycin, tetracycline, chloramphenicol, vancomycin, nitrofurantoin, and aztreonam at the highest concentration of antimicrobial agent present in the microdilution trays. The addition of Tween 80 to 7HSF broth increased the susceptibility of M. avium complex to many of the antimicrobial agents tested. Killing of M. avium complex (i.e., less than or equal to 1% survival after 7 days) was found to vary for different strains and antimicrobial agents. Killing of some strains by amoxicillin-clavulanic acid, carbenicillin, azlocillin, cefmenoxime, cefotaxime, amikacin, and ampicillin occurred at concentrations of antimicrobial agent that are achievable in serum. Further studies are needed to determine whether any of these antimicrobial agents has activity against M. avium complex cells that have been ingested by macrophages.