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The antibiotic resistance of Mycoplasma mycoides ssp. mycoides strain T1 was investigated. This strain was resistant to high levels (greater than 100 micrograms ml-1) of rifampicin and nalidixic acid. It was sensitive to streptomycin, spectinomycin and novobiocin; however, single step mutants with high levels of resistance (greater than 100 micrograms ml-1) were readily isolated. With erythromycin and tylosin for which the minimum inhibitory concentration (MIC) for the parent strain was less than 0.1 microgram ml-1, mutants resistant to greater than 100 micrograms ml-1 were obtained in two and three steps respectively. The MIC of tetracycline in single step resistant mutants (0.6 microgram ml-1) was tenfold higher than the parent strain, but could not be increased further. There was only a twofold increase in resistance to chloramphenicol in single step mutants. The frequency of resistant mutants varied with the antibiotic and was between 4 X 10(-6) and 2 X 10(-8). The mutation rate to antibiotic resistance to streptomycin, spectinomycin, novobiocin, erythromycin and tylosin was between 3 X 10(-8) and 5 X 10(-9) per cell per generation. There was a fivefold decrease in mutation rate to resistance to 60 micrograms ml-1 streptomycin compared to that to 20 micrograms ml-1.

Ureaplasma urealyticum organisms (ureaplasmas) were isolated in large numbers (up to 10(8) colour changing units (ccu)/ml) over a period of four years from the urethra of a man with hypogammaglobulinaemia and non-gonococcal urethritis. Elimination of Mycoplasma hominis by antibiotic treatment early in the course of the urethritis did not diminish the severity of his condition, which indicated that this mycoplasma was not a cause. Courses of treatment with tetracyclines, spectinomycin, erythromycin, rosaramicin, and clindamycin on each occasion reduced the numbers of ureaplasma isolated from the urethra and the severity of disease. The organisms were not eliminated, however, sometimes due to the development of antibiotic resistance, and the urethritis recurred. Though netilmicin was not particularly effective in vitro, it was effective clinically, the disease resolving and the organisms disappearing for five months. Recurrence of urethritis, accompanied by epididymitis, was associated this time with the recovery of a different (tetracycline sensitive) ureaplasma strain; the urethritis and epididymitis were treated successfully with a combination of netilmicin and doxycycline. The administration of ureaplasma antiserum did not seem to be instrumental in eradicating the ureaplasmas. The various antibiotics had a greater influence on the mycoplasmas in the urethra than on those in the throat and joints, perhaps because the antibiotics were concentrated in the urogenital tract. The close association between the occurrence of urethritis and the ureaplasmas suggests strongly that they were responsible for it.

The susceptibilities of T-mycoplasmas (Ureaplasma urealyticum) to minocycline, demeclocycline, doxycycline, tetracycline, and erythromycin were determined by a direct tube dilution test. T-mycoplasma-positive urine sediments of 105 patients with a history of reproductive failure were used as inocula. Minocycline was found to be the most active of the group of antibiotics commonly used to eradicate T-mycoplasma infection. Based on the median initial minimum inhibitory concentration, minocycline was the lowest with 0.03 μg/ml, followed by demeclocycline and doxycycline with 0.125 μg/ml, tetracycline with 0.25 μg/ml, and erythromycin with 2.0 μg/ml. Six T-mycoplasma isolates which had been cloned three times were also tested for susceptibility to the same five antibiotics. The same susceptibility pattern was found. Strains resistant to high concentrations of all antibiotics occurred. Strong positive correlation was seen in 21 patients between in vitro highly resistant strains and positive posttreatment cultures. These results indicate that empirical treatment of genital mycoplasma infections is not justified. Cultures should be taken pretreatment, susceptibility testing performed prior to treatment, and follow-up cultures done posttreatment.

A combination of lincomycin-spectinomycin-tylosin was tested against several strains of mycoplasmas and acholeplasmas as might be encountered in bovine semen and shown to be effective against them. This combination as well as minocin , rosaramicin, rosoxacin, tiamulin, gentamicin and declomycin were tested in vitro against 58 isolates of ureaplasma from the bovine urogenital tract. The lincomycin-spectinomycin-tylosin combination, minocin , rosaramicin, tiamulin and declomycin were quite active, while rosoxacin and gentamicin were much less active against the test strains.

The purpose of this study was to determine the susceptibility of various strains of Mycoplasma bovis, Mycoplasma dispar, and Ureaplasma diversum, which are prevalent causes of pneumonia in calves, to 16 antimicrobial agents in vitro. The MICs of the antimicrobial agents were determined by a serial broth dilution method for 16 field strains and the type strain of M. bovis, for 19 field strains and the type strain of M. dispar, and for 17 field strains of U. diversum. Final MICs for M. bovis and M. dispar were read after 7 days and final MICs for U. diversum after 1 to 2 days. All strains tested were susceptible to tylosin, kitasamycin, and tiamulin but were resistant to nifuroquine and streptomycin. Most strains of U. diversum were intermediately susceptible to oxytetracycline but fully susceptible to chlortetracycline; most strains of M. bovis and M. dispar, however, were resistant to both agents. Strains of M. dispar and U. diversum were susceptible to doxycycline and minocycline, but strains of M. bovis were only intermediately susceptible. Susceptibility or resistance to chloramphenicol, spiramycin, spectinomycin, lincomycin, or enrofloxacin depended on the species but was not equal for the three species. The type strains of M. bovis and M. dispar were more susceptible to various antimicrobial agents, including tetracyclines, than the field strains. This finding might indicate that M. bovis and M. dispar strains are becoming resistant to these agents. Antimicrobial agents that are effective in vitro against all three mycoplasma species can be considered for treating mycoplasma infections in pneumonic calves. Therefore, tylosin, kitasamycin, and tiamulin may be preferred over oxytetracycline and chlortetracycline.

Bull semen is commonly contaminated with mycoplasmas. To determine the source of contamination, semen and the genital tracts of 45 artificial insemination bulls were cultured for these organisms. The results indicate that mycoplasmas colonize the prepuce and the distal part of the urethra. Only rarely were they found in the ampullae or seminal vesicles. In 92% of the bulls with contaminated semen the same Mycoplasma species or Ureaplasma diversum was isolated from the prepuce and urethral orifice as was found in the semen. This suggests that the prepuce and distal urethra is the source of contamination. Colonization of the genital tracts with Mycoplasmas or U. diversum was not associated with histological changes.

The increasing resistance of genital mycoplasmas to tetracycline poses a problem because tetracycline is one of the few antimicrobial agents active against Mycoplasma hominis, Ureaplasma urealyticum, chlamydiae, gonococci, and other agents of genitourinary-tract disease. Since the quinolones are a promising group of antimicrobial agents, the susceptibilities of M. hominis and U. urealyticum to the newer 6-fluoroquinolones were determined by the agar dilution method. Ciprofloxacin, difloxacin, and ofloxacin had good activity against M. hominis, with the MIC for 50% of isolates tested (MIC50) being 1 microgram/ml. Fleroxacin, lomefloxacin, pefloxacin, and rosoxacin had MIC50s of 2 micrograms/ml. Enoxacin, norfloxacin, and amifloxacin had MIC50s of 8 to 16 micrograms/ml, and cinoxacin and nalidixic acid were inactive (MIC50, greater than or equal to 256 micrograms/ml). Overall, the activities of 6-fluoroquinolones for ureaplasmas were similar to those for M. hominis, with MICs being the same or twofold greater. The most active 6-fluoroquinolones against ureaplasmas were difloxacin, ofloxacin, and pefloxacin, with MIC50s of 1 to 2 micrograms/ml. Ciprofloxacin was unusual in that the MIC50 for M. hominis was 1 microgram/ml, whereas the MIC50 for ureaplasmas was 8 micrograms/ml. Since the MIC50s for the most active quinolones approximate achievable concentrations in blood and urine, quinolones have promise in treating mycoplasmal infections.

We report herein a survey in which cultures of bovine reproductive tracts for Ureaplasma diversum and mycoplasmas were carried out in order to better understand the role of these organisms in granular vulvitis (GV). Samples cultured were vulvar swabs from clinically normal cows or ones with GV, preputial swabs or raw semen from bulls, and abomasal contents of aborted fetuses.

Ureaplasma diversum was isolated from 104 (43.3%) of 240 dairy cows, 32 (27.1%) of 118 beef cows, 43 (47.2%) of 91 beef heifers, 23 (67.6%) of 34 beef bulls, and three (60%) of five dairy bulls. Mycoplasmas were isolated from 18 (7.5%) dairy cows, two (1.6%) beef cows, three (8.8%) beef bulls, and one dairy bull. No isolation was made from 97 aborted fetuses. For 65 dairy cows and 30 beef heifers with vulvar lesions, the isolation rates for ureaplasmas of 62.5% and 69.7%, respectively, were significantly higher (X2) than those for normal animals (37.5% and 30.3%). On immunofluorescent serotyping of 137 of the 205 isolates, there were 66 in serogroup C (strain T44), 18 in serogroup B (strain D48), eight in serogroup A (strain A417 or strain 2312), 14 cross-reacting, and 31 that were not identified. It was concluded that U. diversum is commonly present in the lower reproductive tract of beef/dairy cattle in Saskatchewan and is associated with granular vulvitis.

Agar dilution antimicrobial susceptibility testing of Camphylobacter jejuni showed that erythromycin, clindamycin, nitrofurantoin, and gentamicin were the most active compounds, inhibiting 90% of the isolates at a concentration of 1 microgram/ml or less. The frequency of high-level erythromycin resistance was 1%. Erythromycin-resistant isolates showed cross-resistance to clindamycin. All strains were inhibited by chloramphenicol at less than or equal to 8 micrograms/ml. About 20% of the isolates were resistant to tetracycline at 4 micrograms/ml. All strains were highly resistant to novobiocin, bacitracin, vancomycin, and trimethoprim and resistant to rifampin. The minimal inhibitory concentrations (MICs) of metronidazole ranged from less than or equal to 0.5 to 128 micrograms/ml. The susceptibility of strains to sulfamethoxazole and polymyxin B sulfate was markedly influenced by inoculum size. The MICs of polymyxin B sulfate were significantly higher at 42 than 36 degrees C. All strains were inhibited by nalidixic acid at 32 micrograms/ml. In the penicillin group, ampicillin was the most active compound, inhibiting only about three-quarters of the strains at 8 micrograms/ml. The cephalosporins as a group showed only moderate to poor activity, the most active cephalosporin being cefotaxime, which inhibited about 90% of the strains at 8 micrograms/ml. The use of antibiotics in selective media is discussed.

The flurofamide sensitivities of 21 bovine ureaplasma isolates were determined using the metabolic inhibition method. The 21 isolates included seven each of vaginal, preputial and seminal origin. The minimum inhibitory concentrations of flurofamide ranged from 0.0125 to 0.2 meg/mL against a ureaplasma titer of 10(4) organisms/mL. The minimum lethal concentrations ranged from less than or equal to 0.1 to 3.2 mcg/mL. Flurofamide was then evaluated in a system comparable to the procedure for semen extension with respect to temperature, time and dilution. The compound was found to be ineffective in reducing ureaplasma numbers in this system at levels up to 1500 mcg/mL.

A group of 150 Staphylococcus saprophyticus strains isolated from urinary tract infections in women were included in this study. Antimicrobial susceptibility tests showed that these isolates were sensitive to most antimicrobial agents. All strains were sensitive to penicillin, cephalothin, gentamicin, kanamycin, trimethoprim and nitrofurantoin. Resistance to tetracycline was present in 10.6% of the strains, to chloramphenicol in 4%, to erythromycin in 1.3% and to streptomycin in 1.3%. All strains were resistant to cadmium chloride as well as to novobiocin and nalidixic acid. Plasmid analysis showed that 82% of the strains harboured plasmids, some of them with complex plasmid profiles. Most plasmids were considered to be cryptic, although antibiotic resistance plasmids were identified in 18 isolates. Tetracycline resistance was encoded by a plasmid of c. 2.8 MDa, chloramphenicol resistance by a plasmid of c. 2.9 MDa and erythromycin resistance by a plasmid of c. 1.6 MDa. Streptomycin resistance could not be linked to the presence of any specific plasmid. Plasmid profiling seemed to be a good method for differentiating among S. saprophyticus strains.

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Infection, lesions and clinical significance of Acheloplasmas, Mycoplasma bovis and Mycoplasma bovigenitalium in genital disease of cattle are described. A more detailed account is given of ureaplasma infections. Acute and chronic forms of granular vulvitis in both field and experimental disease are described as well as the role of the organism in abortion.

Recovery rates of ureaplasma and mycoplasma from semen and preputial washings in bulls are outlined and their significance in disease is discussed. There are problems in differentiating pathogenic from nonpathogenic isolates. Methods are being developed to treat semen for these organisms.

This paper provides a concise summary of clinical and microbiological aspects of bovine genital mycoplasmosis.

The criteria that need to be fulfilled before regarding a mycoplasma as a cause of non-gonococcal urethritis (NGU) are outlined. Of the seven mycoplasmas that have been isolated from the human genitourinary tract, most cannot be considered as contenders for causing NGU. Although there is no evidence to support an etiological role for Mycoplasma hominis, it may be unwise to ignore this mycoplasma in view of its known pathogenicity in other situations. The cumulative weight of evidence indicates that strains of Ureaplasma urealyticum (ureaplasmas) cause NGU in some patients. The reason for their occurrence in the urethra of some men without disease needs to be established. Ureaplasmas do not seem to cause post-gonococcal urethritis. The role in NGU of M. genitalium, newly discovered in the male urethra, is unknown, but its biological features, morphological appearance, and ability to cause genital disease in marmosets suggest that it may be pathogenic for man.

The susceptibility of 11 T-strains, 12 strains of Mycoplasma hominis, and a single strain of M. fermentans to 15 antimicrobial agents was determined by study of inhibition of metabolic activity in a broth dilution system. All three species were inhibited by tetracycline, chloramphenicol, streptomycin, gentamicin, and kanamycin, and were relatively resistant to cephalothin, cephaloridine, polymyxin, vancomycin, and ampicillin. Three antimicrobial agents had significant differential effects on these species. Erythromycin was more active against T-strains than against M. hominis or M. fermentans. Lincomycin, clindamycin, and nitrofurantoin had greater activity against M. hominis and M. fermentans than against T-strains. The activity of the drugs tested was generally uniform over a wide range of inocula. The effect of pH and the difference between minimal inhibiting and minimal mycoplasmacidal concentrations of the drugs tested were consistent with expectations based on the effects of these drugs on bacteria.

During a two-year period of observation Bacteroides species were isolated from specimens of pus and vaginal swabs from 115 patients in this hospital. Thirty-five representative strains proved on examination to be Bacteroides fragilis.

Minimal inhibitory and minimal bactericidal concentrations of six antibiotics for these strains were determined. All strains were resistant to streptomycin, neomycin, and polymyxin, slightly sensitive to penicillin and ampicillin, and fully sensitive to tetracycline, chloramphenicol, erythromycin, and lincomycin. The minimum bactericidal concentrations of chloramphenicol, erythromycin, and lincomycin were two to four times the minimal inhibitory concentrations. Tetracycline failed to exert any consistent bactericidal effect.

The treatment of patients with infections caused by B. fragilis is discussed in the light of the findings in vitro.

A simple, direct broth-disk test, utilizing urine sediment as the inoculum and impregnated paper disks as the source of antibiotic, was developed and used to test the susceptibility of 54 isolates of Ureaplasma urealyticum (T-strain mycoplasmas) to minocycline, doxycycline, demeclocycline, tetracycline, and erythromycin. The concentration of each antibiotic was calculated to approximate the attainable blood level. Resistance or susceptibility to each antibiotic was determined by growth, indicated by a color change of the medium in each tube, comparable to that of a control culture without antibiotic. Of the 54 T-mycoplasmas tested, 46 (85.2%) were inhibited by 1 μg of minocycline per ml, 45 (83.3%) were inhibited by 1 μg of doxycycline per ml, 38 (72.2%) were inhibited by 1 μg of demeclocycline per ml, 18 (33.3%) were inhibited by 1 μg of tetracycline per ml, and only 2 (3.7%) were inhibited by 3 μg of erythromycin per ml. Seven (13%) of the 54 T-mycoplasmas tested were resistant to all five antibiotics. There was good correlation between results obtained by this direct broth-disk method and minimal inhibitory concentrations obtained by the direct broth dilution method.

Objective: Our goal was to determine the in vitro susceptibility of Ureaplasma urealyticum and Mycoplasma hominis isolates to several antibiotics in Argentina.

Methods: Ninety-four strains of U. urealyticum and 18 strains of M. hominis isolated from cervical and urethral specimens were studied. Broth microdilution and agar dilution tests for minocycline, tetracycline, erythromycin, ciprofloxacin, and ofloxacin were performed.

Results: Both methods proved to be reliable and reproducible for U. urealyticum and M. hominis, with no major differences in results. The U. urealyticurn strains were inhibited by erythromycin at MICs ranging from ≤0.5 to >8 μ/ml. Ofloxacin showed the highest activity against this latter organism. No differences between tetracycline and minocycline MICs were observed with U. urealyticum. Two M. hominis strains displaying high MICs both to tetracycline and to minocycline were detected.

Conclusions: The emerging resistance of mycoplasmas to certain antibiotics emphasizes the need to undertake further surveillance studies on the clinical isolates of such organisms.

A bioassay was developed to measure the minimum concentration of an antimicrobial drug that disrupts the colonization resistance mediated by model human intestinal microbiota against Salmonella invasion of Caco-2 intestinal cells. The bioassay was used to measure the minimum disruptive concentrations (MDCs) of drugs used in animal agriculture. The MDCs varied from 0.125 μg/ml for some broad-spectrum antimicrobial drugs (e.g., streptomycin) to 16 μg/ml for drugs with limited spectra of antimicrobial activity (e.g., lincomycin). The acceptable daily intake (ADI) residue concentration calculated on the basis of the MDCs were higher for erythromycin, lincomycin, and tylosin than the ADI residue concentrations calculated on the basis of the MICs. The MDC-based ADI values for apramycin, bacitracin, neomycin, novobiocin, penicillin G, streptomycin, tetracycline, and vancomycin were lower than the reported MIC-based ADI values. The effects of antimicrobial drugs at their MDCs on the bacterial composition of the microbiota were observed by denaturing gradient gel electrophoresis of 16S rRNA sequences amplified by PCR. Changes in the population composition of the model colonization resistance microbiota occurred simultaneously with reduced colonization resistance. The results of this study suggest that direct assessment of the effects of antimicrobial drugs on colonization resistance in an in vitro model can be useful in determining ADI values.

In vitro minimum inhibitory concentrations were determined for 21 antimicrobials against 41 isolates of Mycoplasma mycoides subsp. mycoides small-colony type, the cause of contagious bovine pleuropneumonia. Of the antimicrobials used most widely in Africa, oxytetracycline and tilmicosin were effective, while the isolates were resistant to tylosin. These results provide a baseline for monitoring antimicrobial resistance.

Fifty strains of Streptococcus mutans, including defined strains and clinical isolates, were tested for susceptibility to 20 different antimicrobial agents. Minimal inhibitory concentrations were determined by a liquid microtiter procedure. Antibiotics that were most effective in concentrations below 0.1 μg/ml included penicillin, ampicillin, erythromycin, cephalothin, and methicillin. Antibiotics effective in concentrations between 0.1 μg and 10 μg/ml included rifampin, lincomycin, thiostrepton, spiromycin, vancomycin, streptolydigan, novobiocin, tetracycline, chloramphenicol, spectinomycin, and gentamicin. Antibiotics effective at higher concentrations ranging from 10 μg/ml to 400 μg/ml included the aminoglycosides kanamycin, neomycin, streptomycin, and kasugamycin. Although most antibiotics exhibited inhibitory effects in a narrow range of concentrations, antibiotics such as tetracycline, thiostrepton, and spiromycin had a 1,000-fold range from the lowest to highest concentrations required for growth inhibition.

The sites in the genital tract from which mycoplasmas could be recovered at various stages of the estrous cycle were studied in five Standardbred mares naturally infected with Mycoplasma. Mycoplasma equigenitalium and Mycoplasma subdolum were most frequently isolated from the clitoral fossa as compared to the vagina, cervix, and uterus. The lowest isolation prevalence was observed in the uterus. The recovery of Mycoplasma spp. from the clitoral fossa did not differ at any stage of the estrous cycle; however, recovery from the vagina, cervix, and uterus was variable during the cycle and more organisms were recovered on the day of ovulation than at any other time. From these results it was concluded that the clitoral fossa is the most likely “ecological niche” for Mycoplasma spp. in the mare. Ureaplasmas were not isolated.

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Objective: The involvement of the genital mycoplasmas Ureaplasma urealyticum and Mycoplasma hominis in complications of pregnancy has remained controversial especially because these microorganisms are frequent colonizers of the lower genital tract. Recovery of bacteria from the placenta appears to be the sole technique to represent a true infection and not vaginal contamination. Therefore, we investigated the presence of genital mycoplasmas, aerobic and anaerobic bacteria, and fungi in human placentas and evaluated their association with morbidity and mortality of pregnancy.

Methods: We cultured placentas from 82 women with complicated pregnancies. One hundred placentas from women with uncomplicated pregnancies were evaluated as controls. When possible, placentas were examined histologically for presence of chorioamnionitis.

Results: Microorganisms were recovered from 52% of the placentas of complicated pregnancies and U. urealyticum was the microorganism isolated most frequently from the placenta. A significant association between positive mycoplasma culture of the placenta and complication of pregnancy was found, and chorioamnionitis was positively related to isolation of mycoplasmas.

Conclusions: These data suggest that genital mycoplasmas are able to infect the human placenta where they can cause chorioamnionitis. This infection of the placenta by genital mycoplasmas is related to preterm birth and fatal outcome of pregnancy.

A microtiter technique was used for determination of the sensitivity of Mycoplasma hyosynoviae to antibiotics and other drugs. Use of a biphasic agar-broth medium in microtiter plates allowed direct visualization of growth. Results were more reproducible with this system than when broth alone was used and evaluation based on color change was required. Attempts to adapt the test for use with Mycoplasma hyorhinis were not successful. Minimal inhibitory concentrations of 12 drugs and drug combinations for 12 strains of M. hyosynoviae are presented. Drugs with the lowest minimal inhibitory concentrations were tylosin (0.37 mcg/ml)and lincomycin (0.88 mcg/ml), both of which have been used for treatment and control of M. hyosynoviae arthritis. Comparison of the minimal inhibitory concentrations of tylosin for 43 isolates of M. hyosynoviae obtained in 1959 and 1960 and from 1966 through 1971 indicated the possibilty of decreasing sensitivity to the drug although differences between recent isolates and earlier ones were not statistically significant.

The in vitro activities of two investigational quinolones, sparfloxacin (previously designated AT 4140) and PD 127391, were determined for 30 strains each of Mycoplasma pneumoniae, Mycoplasma hominis, and Ureaplasma urealyticum and compared with those of ciprofloxacin, tetracycline, clindamycin, and erythromycin. Erythromycin was the most active compound against M. pneumoniae (maximum MIC, less than 0.008 microgram/ml). PD 127391 (MICs, less than 0.008 to 0.031 microgram/ml), sparfloxacin (MICs, less than 0.008 to 0.25 microgram/ml), clindamycin (MICs, less than 0.008 to 0.5 microgram/ml), and tetracycline (MICs, 0.063 to 0.25 microgram/ml) were superior to ciprofloxacin (MICs, 0.5 to 2 microgram/ml). Sparfloxacin and PD 127391 were active against M. hominis (MICs, less than 0.008 to 0.031 microgram/ml for each) at concentrations comparable to those of clindamycin (MICs, less than 0.008 to 0.063 microgram/ml) and at concentrations lower than those of ciprofloxacin (MICs, 0.125 to 0.5 microgram/ml). As expected, M. hominis was resistant to erythromycin (MICs, 32 to greater than or equal to 256 micrograms/ml). For U. urealyticum, PD 127391 (MICs, 0.031 to 0.5 microgram/ml) and sparfloxacin (MICs, 0.063 to 1 microgram/ml) were superior to erythromycin (MICs, 0.25 to 4 micrograms/ml), ciprofloxacin (MICs, 0.5 to 8 micrograms/ml), and clindamycin (MICs, 0.25 to 64 micrograms/ml. Both new quinolones were equally active against tetracycline-susceptible as well as resistant strains of M. hominis and U. urealyticum. The possible influence of medium components and/or pH on MICs was evaluated by testing a Staphylococcus aureus reference strain with each antibiotic in SP-4 broth and 10-B broth and comparing the results with published MICs for this strain. MICs determined in 10-B broth for erythromycin were affected most. This study shows that the activities of sparfloxacin and PD 127391 are similar to one another and comparable or superior to those of other drugs used to treat mycoplasmal infections. The MICs of both new quinolones were consistently 2 to several dilutions lower than those of ciprofloxacin for each species.

Three cephalosporin-related antibiotics and nine other antimicrobial agents were studied for in vitro effectiveness against 54 recently isolated strains of Salmonella. Minimal inhibitory concentrations determined by the plate dilution method demonstrated the following percentages of resistance: ampicillin, 6%; tetracycline, 13%; streptomycin, 52%; sulfadiazine, 94%; cephaloglycin, 96%; and lincomycin, 100%. No strains were resistant to cephalothin, cephaloridine, chloramphenicol, colistimethate, kanamycin, and polymyxin B. The commonest serotype studied, S. typhimurium, showed the greatest antibiotic resistance, with 21% resistant to ampicillin, 36% resistant to tetracycline, and 71% resistant to streptomycin. Cephalothin and cephaloridine were highly effective in vitro but inhibitory concentrations of 20 to 40 μg of cephaloglycin per ml were required for the majority of Salmonella strains.