Susceptibility of Neisseria gonorrhoeae to seven antibiotics in vitro

One hundred consecutive isolates of N. gonorrhoeae were tested for susceptibility to penicillin, ampicillin, tetracycline, erythromycin, kanamycin, cephaloridine and cephalexin by an agar dilution method. Relative resistance to penicillin was frequent. For 39% of isolates the minimum inhibitory concentration (MIC) of penicillin was 0.05 U./ml. or less; in 55% the MIC was 0.5 to 2.0 U./ml. Ampicillin was slightly more active than penicillin G: all isolates were inhibited by 0.5μg./ml. or less. Resistance to tetracycline and erythromycin was frequent with MIC of 1 μg./ml. or greater observed in 32 and 24% of isolates respectively. The MIC of kanamycin for all gonococci was 8 μg./ml. or greater. Cephalexin was slightly more active than cephaloridine, though each drug exhibited a wide range of MIC values. Gonococcus isolates resistant to penicillin (MIC of 1.0 U./ml. or greater) tended to be resistant to the other antibiotics tested.     

II. In vitro studies of antibacterial activity

One hundred and forty isolates of beta-hemolytic streptococcus cultured from patients with clinical pharyngitis were studied by disc diffusion for antibiotic sensitivity to lincomycin, erythromycin, cephalexin and penicillin and by agar dilution to cephalexin and penicillin. All isolates were sensitive to ≤ 0.1 μg./ml. penicillin and ≤ 1.56 μg./ml. cephalexin. The disc-diffusion test was reliable in predicting the sensitivities in vitro. One strain of group A betahemolytic streptococcus was resistant to erythromycin by disc diffusion. When compared to Lancefield grouping 18% of strains were incorrectly identified as group A by the bacitracin-disc test. Cephalexin was uniformly effective in vitro in inhibiting beta-hemolytic streptococci and the 30 μg. cephalexin disc was reliable in predicting these sensitivities.     

Persistence of Vancomycin-Resistant Enterococci in New Zealand Broilers after Discontinuation of Avoparcin Use

Large amounts of tylosin, zinc-bacitracin, and avilamycin are currently used as prophylactics in New Zealand broiler production. Avoparcin was also used from 1977 to 2000. A total of 382 enterococci were isolated from 213 fecal samples (147 individual poultry farms) using enrichment broths plated on m-Enterococcus agar lacking antimicrobials. These isolates were then examined to determine the prevalence of antimicrobial resistance. Of the 382 isolates, 5.8% (22 isolates) were resistant to vancomycin, and 64.7% were resistant to erythromycin. The bacitracin MIC was ≥256 μg/ml for 98.7% of isolates, and the avilamycin MIC was ≥8 μg/ml for 14.9% of isolates. No resistance to ampicillin or gentamicin was detected. Of the 22 vancomycin-resistant enterococci (VRE) isolates, 18 (81.8%) were Enterococcus faecalis, 3 were Enterococcus faecium, and 1 was Enterococcus durans. However, when the 213 fecal enrichment broths were plated on m-Enterococcus agar containing vancomycin, 86 VRE were recovered; 66% of these isolates were E. faecium and the remainder were E. faecalis. Vancomycin-resistant E. faecium isolates were found to have heterogenous pulsed-field gel electrophoresis (PFGE) patterns of SmaI-digested DNA, whereas the PFGE patterns of vancomycin-resistant E. faecalis isolates were identical or closely related, suggesting that this VRE clone is widespread throughout New Zealand. These data demonstrate that vancomycin-resistant E. faecalis persists in the absence and presence of vancomycin-selective pressure, thus explaining the dominance of this VRE clone even in the absence of avoparcin.     

Implications of Amino Acid Substitutions in GyrA at Position 83 in Terms of Oxolinic Acid Resistance in Field Isolates of Burkholderia glumae, a Causal Agent of Bacterial Seedling Rot and Grain Rot of Rice

Oxolinic acid (OA), a quinolone, inhibits the activity of DNA gyrase composed of GyrA and GyrB and shows antibacterial activity against Burkholderia glumae. Since B. glumae causes bacterial seedling rot and grain rot of rice, both of which are devastating diseases, the emergence of OA-resistant bacteria has important implications on rice cultivation in Japan. Based on the MIC of OA, 35 B. glumae field isolates isolated from rice seedlings grown from OA-treated seeds in Japan were divided into sensitive isolates (OSs; 0.5 μg/ml), moderately resistant isolates (MRs; 50 μg/ml), and highly resistant isolates (HRs; ≥100 μg/ml). Recombination with gyrA of an OS, Pg-10, led MRs and HRs to become OA susceptible, suggesting that gyrA mutations are involved in the OA resistance of field isolates. The amino acid at position 83 in the GyrA of all OSs was Ser, but in all MRs and HRs it was Arg and Ile, respectively. Ser83Arg and Ser83Ile substitutions in the GyrA of an OS, Pg-10, resulted in moderate and high OA resistance, respectively. Moreover, Arg83Ser and Ile83Ser substitutions in the GyrA of MRs and HRs, respectively, resulted in susceptibility to OA. These results suggest that Ser83Arg and Ser83Ile substitutions in GyrA are commonly responsible for resistance to OA in B. glumae field isolates.     

Unclassified Mycobacteria Isolated from Human Suspect Tuberculosis Cases in Newfoundland: Preliminary Studies on Fifteen Strains

Unclassified mycobacteria were isolated from 36 of 35,555 clinical specimens cultured for M. tuberculosis. The majority of isolations were from patients suspected of having tuberculosis and from whom repeat attempts at culture failed to yield typical tubercle bacilli. Fifteen strains thus far studied were not capable of causing generalized tuberculosis in guinea pigs, and all were highly resistant to the commonly employed antituberculous therapeutic agents. Eleven of the 15 strains were resistant to 100 or more μg./ml. of streptomycin; 12 strains were resistant to 25 or more μg./ml. of para - aminosalicylic acid; and all 15 showed growth in the presence of 50 or more μg./ml. of isoniazid. All strains were niacin-negative and catalase-positive. In the absence of other cultural findings, isolation of anonymous mycobacteria poses a major problem, especially in those cases in which the clinical and radiographic findings are typically those of tuberculosis.     

In Vitro Susceptibility of Atypical Mycobacteria To Rifampin

Atypical mycobacteria (209 strains) were examined for susceptibility to rifampin by the proportion method by using Middlebrook 7H-10 agar. All strains of Mycobacterium kansasii and tap-water scotochromogens were inhibited by 0.25 to 1 μg of the drug per ml. Seventy-six per cent of M. scrofulaceum and 61% of M. intracellulare strains were susceptible to 4 μg/ml or less; 5% of the former and 8% of the latter were resistant to 16 μg/ml. All strains of M. gastri and M. triviale and most strains of M. terrae were sensitive to 1 to 4 μg/ml. Two strains of M. borstelense were both inhibited by 8 μg/ml. Nearly all strains of M. fortuitum were resistant to the drug. The results of this study suggest that rifampin may be a valuable agent for the treatment of many atypical mycobacterial infections.     

In Vitro Studies of α-Carboxyl-3-Thienylmethyl Penicillin,a New Semisynthetic Penicillin

The activity of a new semisynthetic penicillin, α-carboxyl-3-thienylmethyl penicillin (BRL-2288) was determined against 535 clinical isolates of gram-negative bacilli, by using the tube dilution technique. Nearly 80% of isolates of Proteus spp. were inhibited by 3.12 μg or less of this antibiotic per ml. BRL-2288 was as active as ampicillin against Escherichia coli. It was slightly more active than carbenicillin or 6-(d-α-sulfoaminophenylacetamido)-penicillanic acid against Pseudomonas sp., with over half of the isolates being inhibited by 50 μg or less of BRL-2288 per ml. Isolates of Klebsiella sp. were routinely resistant to this antibiotic. The drug was bactericidal against most sensitive organisms. BRL-2288 was less active against large inocula. A strain of Pseudomonas sp. which developed resistance to carbenicillin also developed resistance to BRL-2288 simultaneously.     

Role of Calf-Adapted Escherichia coli in Maintenance of Antimicrobial Drug Resistance in Dairy Calves

The prevalence of antimicrobial drug-resistant bacteria is typically highest in younger animals, and prevalence is not necessarily related to recent use of antimicrobial drugs. In dairy cattle, we hypothesize that antimicrobial drug-resistant, neonate-adapted bacteria are responsible for the observed high frequencies of resistant Escherichia coli in calves. To explore this issue, we examined the age distribution of antimicrobial drug-resistant E. coli from Holstein cattle at a local dairy and conducted an experiment to determine if low doses of oxytetracycline affected the prevalence of antimicrobial drug-resistant E. coli. Isolates resistant to tetracycline (>4 μg/ml) were more prevalent in <3-month-old calves (79%) compared with lactating cows (14%). In an experimental trial where calves received diets supplemented with or without oxytetracycline, the prevalence of tetracycline-resistant E. coli was slightly higher for the latter group (P = 0.039), indicating that drug use was not required to maintain a high prevalence of resistant E. coli. The most common resistance pattern among calf E. coli isolates included resistance to streptomycin (>12 μg/ml), sulfadiazine (>512 μg/ml), and tetracycline (>4 μg/ml) (SSuT), and this resistance pattern was most prevalent during the period when calves were on milk diets. To determine if prevalence was a function of differential fitness, we orally inoculated animals with nalidixic acid-resistant strains of SSuT E. coli and susceptible E. coli. Shedding of SSuT E. coli was significantly greater than that of susceptible strains in neonatal calves (P < 0.001), whereas there was no difference in older animals (P = 0.5). These data support the hypothesis that active selection for traits linked to the SSuT phenotype are responsible for maintaining drug-resistant E. coli in this population of dairy calves.     

Nicotine Content of Tobacco Roots and Toxicity to Meloidogyne incognita

The motility of Meloidogyne incognita second-stage juveniles (J2) and their ability to induce root galls in tomato were progressively decreased upon exposure to nicotine at concentrations of 1-100 μg/ml. EC₅₀ values ranged from 14.5 to 22.3 μg/ml, but J2 motility and root-gall induction were not eliminated at 100 μg/ml nicotine. Nicotine in both resistant NC 89 and susceptible NC 2326 tobacco roots was increased significantly 4 days after exposure to M. incognita. The increase was greater in resistant than in susceptible tobacco. Root nicotine concentrations were estimated to be 661.1-979.1 μg/g fresh weight. More M. incognita were detected in roots of susceptible than in roots of resistant tobacco. Numbers of nematodes within resistant roots decreased as duration of exposure to M. incognita was increased from 4 to 16 days. Concentrations of nicotine were apparently sufficient to affect M. incognita in both susceptible and resistant tobacco roots. Localization of nicotine at infection sites must be determined to ascertain its association with resistance.     

Effects of Orally Administered Tetracycline on the Intestinal Community Structure of Chickens and on tet Determinant Carriage by Commensal Bacteria and Campylobacter jejuni

There is a growing concern that antibiotic usage in animal production has selected for resistant food-borne bacteria. Since tetracyclines are common therapeutic antibiotics used in poultry production, we sought to evaluate the effects of oral administration on the resistance of poultry commensal bacteria and the intestinal bacterial community structure. The diversity indices calculated from terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA amplicons did not indicate significant changes in the cecal bacterial community in response to oxytetracycline. To evaluate its effects on cultivable commensals, Enterococcus spp., Escherichia coli, and Campylobacter spp. were isolated from the cecal droppings of broiler chickens. Enterococcus spp. and E. coli expressed tetracycline MICs of >8 μg/ml and harbored a variety of tet resistance determinants regardless of the tetracycline exposure history of the birds. The enterococcal isolates possessed tetM (61%), tetL (25.4%), and tetK (1.3%), as well as tetO (52.5%), the determinant known to confer a tetracycline resistance phenotype in Campylobacter jejuni. E. coli isolates harbored tetA (32.2%) or tetB (30.5%). Tetracycline MICs remained at <2 μg/ml for Campylobacter isolates before and after tetracycline treatment of the chickens, even though isolates expressing MICs of >16 μg/ml were commonly cultured from flocks that did not receive oxytetracycline. The results imply that complex ecological and genetic factors contribute to the prevalence of antibiotic resistance arising from resistance gene transfer in the production environment.     

In Vitro Studies of a New Semisynthetic Penicillin, 6-(D-α-Sulfoaminophenylacetamido)-Penicillanic Acid

The activity of 6-(D-α-sulfoaminophenylacetamido)-penicillanic acid was determined against 357 clinical isolates of gram-negative bacilli by use of the tube-dilution technique. The majority of the isolates of Pseudomonas species were inhibited by 200 μg/ml or less of this antibiotic. Most of the isolates of Escherichia coli had a minimal inhibitory concentration of 50 μg/ml or less. Seventy-three per cent of the isolates of P. mirabilis, 40% of the isolates of P. morganii, and 45% of the isolates of Enterobacter species were inhibited by 12.5 μg/ml or less, whereas most of the isolates of Klebsiella species and Serratia species were resistant. The activity of this semisynthetic penicillin was affected by the size of the inoculum. The drug was bactericidal against all isolates of E. coli and Proteus species that were sensitive to it, but it was bactericidal against only 32% of the sensitive isolates of Pseudomonas species.     

Chromosomal Gene Inactivation in the Green Sulfur Bacterium Chlorobium tepidum by Natural Transformation

Conditions for inactivating chromosomal genes of Chlorobium tepidum by natural transformation and homologous recombination were established. As a model, mutants unable to perform nitrogen fixation were constructed by interrupting nifD with various antibiotic resistance markers. Growth of wild-type C. tepidum at 40°C on agar plates could be completely inhibited by 100 μg of gentamicin ml⁻¹, 2 μg of erythromycin ml⁻¹, 30 μg of chloramphenicol ml⁻¹, or 1 μg of tetracycline ml⁻¹ or a combination of 300 μg of streptomycin ml⁻¹ and 150 μg of spectinomycin ml⁻¹. Transformation was performed by spotting cells and DNA on an agar plate for 10 to 20 h. Transformation frequencies on the order of 10⁻⁷ were observed with gentamicin and erythromycin markers, and transformation frequencies on the order of 10⁻³ were observed with a streptomycin-spectinomycin marker. The frequency of spontaneous mutants resistant to gentamicin, erythromycin, or spectinomycin-streptomycin was undetectable or significantly lower than the transformation frequency. Transformation with the gentamicin marker was observed when the transforming DNA contained 1 or 3 kb of total homologous flanking sequence but not when the transforming DNA contained only 0.3 kb of homologous sequence. Linearized plasmids transformed at least an order of magnitude better than circular plasmids. This work forms a foundation for the systematic targeted inactivation of genes in C. tepidum, whose 2.15-Mb genome has recently been completely sequenced.     

Evaluation of the In Vitro Activity of Tobramycin as Compared with That of Gentamicin Sulfate

The in vitro activity of tobramycin was quantitatively compared with that of gentamicin sulfate against 195 bacterial isolates from clinical material. Tobramycin was found to be twice as active as gentamicin against isolates of Pseudomonas aeruginosa. Conversely, gentamicin proved fourfold more active than tobramycin against isolates of Serratia marcescens. Both drugs were of comparable activity against isolates of Staphylococcus aureus and the majority of the enterobacterial isolates other than S. marcescens. On the basis of the obtained data, the following criteria are proposed for the interpretation of diffusion susceptibility tests with 10-μg discs of gentamicin and tobramycin. Enterobacteriaceae and isolates of S. aureus are designated as susceptible to gentamicin and tobramycin if the zones of inhibition measure 15 mm or more in diameter; zones of 14 mm or less are indicative of resistance. Pseudomonadaceae are interpreted as sensitive to tobramycin and gentamicin if the inhibition zones measure at least 15 and 12 mm in diameter, respectively.     

Resistance of Escherichia coli to tetracyclines. Changes in permeability to tetracyclines in Escherichia coli bearing transferable resistance factors

1. When strains of Escherichia coli, bearing transferable factors for resistance to the tetracyclines (R-factors), and previously cultured in the absence of the tetracyclines, are grown for 15–30min. in a low, subinhibitory, concentration (10μg./ml.) of oxytetracycline or tetracycline, there is a rapid and striking increase in resistance to oxytetracycline or tetracycline, this being associated with a marked fall in the absorption of the drug by the cells. 2. Very short preincubation (1min.) with oxytetracycline, followed by growth for 15–30min. in drug-free medium, produces a marked fall in the absorption of the drug by the resistant cells. Preincubation for 30min. with very low concentrations (0·05μg./ml.) of oxytetracycline produces a similar effect. 3. β-Apo-oxytetracycline, which has very little antibacterial activity, also induces a decreased absorption of oxytetracycline. 4. The ability to exclude oxytetracycline is retained by preincubated resistant cells after growth for 2hr. in drug-free medium. However, after growth for 16hr. in drug-free medium, the cells absorb oxytetracycline freely. 5. Chloramphenicol and proflavine inhibit the adaptive decrease in tetracycline absorption. 5-Fluorouracil has only a slight effect. 6. Spheroplasts prepared from resistant cells show an impaired response to preincubation with tetracycline, compared with intact cells. 7. The relevance of these results to the probable mechanism of tetracycline resistance in R-factor-bearing E. coli is discussed.     

Impact of Three Ampicillin Dosage Regimens on Selection of Ampicillin Resistance in Enterobacteriaceae and Excretion of blaTEM Genes in Swine Feces

The aim of this study was to assess the impact of three ampicillin dosage regimens on ampicillin resistance among Enterobacteriaceae recovered from swine feces by use of phenotypic and genotypic approaches. Phenotypically, ampicillin resistance was determined from the percentage of resistant Enterobacteriaceae and MICs of Escherichia coli isolates. The pool of ampicillin resistance genes was also monitored by quantification of bla_TEM genes, which code for the most frequently produced β-lactamases in gram-negative bacteria, using a newly developed real-time PCR assay. Ampicillin was administered intramuscularly and orally to fed or fasted pigs for 7 days at 20 mg/kg of body weight. The average percentage of resistant Enterobacteriaceae before treatment was between 2.5% and 12%, and bla_TEM gene quantities were below 10⁷ copies/g of feces. By days 4 and 7, the percentage of resistant Enterobacteriaceae exceeded 50% in all treated groups, with some highly resistant strains (MIC of >256 μg/ml). In the control group, bla_TEM gene quantities fluctuated between 10⁴ and 10⁶ copies/g of feces, whereas they fluctuated between 10⁶ to 10⁸ and 10⁷ to 10⁹ copies/g of feces for the intramuscular and oral routes, respectively. Whereas phenotypic evaluations did not discriminate among the three ampicillin dosage regimens, bla_TEM gene quantification was able to differentiate between the effects of two routes of ampicillin administration. Our results suggest that fecal bla_TEM gene quantification provides a sensitive tool to evaluate the impact of ampicillin administration on the selection of ampicillin resistance in the digestive microflora and its dissemination in the environment.     

A combination of sbmA and tolC mutations in Escherichia coli K-12 Tn10-carrying strains results in hypersusceptibility to tetracycline

Previously, we demonstrated that Escherichia coli tolC mutations reduce the high-level resistance to tetracycline afforded by the transposon Tn10-encoded TetA pump from resistance at 200 μg/ml to resistance at 40 μg/ml. In this study, we found that the addition of an sbmA mutation to a tolC::Tn10 mutant exacerbates this phenotype: the double mutant did not form colonies, even in the presence of tetracycline at a concentration as low as 5 μg/ml. Inactivation of sbmA alone partially inhibited high-level tetracycline resistance, from resistance at 200 μg/ml to resistance at 120 μg/ml. There thus appears to be an additive effect of the mutations, resulting in almost complete suppression of the phenotypic expression of Tn10 tetracycline resistance.     

In vitro Activity of Gatifloxacin Against 238 Strains of Anaerobic Bacteria

The activity of gatifloxacin, a new 8-methoxy-fluoroquinolone, was tested against 208 pulmonary pathogens and against an additional 30 isolates of the Bacteroides fragilis group. Pulmonary isolates were from patients with documented anaerobic pleuropulmonary infections and were obtained by appropriate sampling methods. MICs were determined using the NCCLS-approved Wadsworth brucella laked blood agar method and compared to those of clindamycin, imipenem, metronidazole and trovafloxacin. Breakpoints used to define susceptible and [resistant] categories were (in μg/ml): Clindamycin-2, imipenem-4, metronidazole 8 and trovafloxacin. No breakpoint has been defined for gatifloxacin. Gatifloxacin inhibited 99% of all anaerobes tested at 4 μg/ml and 97% of all strains at 2 μg/ml. One strain of B. fragilis was resistant to gatifloxacin at 4 μg/ml; all strains of other B. fragilis group species were susceptible. One strain of Peptostreptococcus sp. was resistant to both gatifloxacin and trovafloxacin (MIC >4 μg/ml). All other strains were susceptible to all agents at ≤μg/ml. All of the non-sporeforming Gram-positive rods were susceptible to gatifloxacin at ≤μg/ml (three strains had an MIC of 4 μg/ml). Trovafloxacin had MICs of 4 μg/ml for two strains, and an MIC of 8 μg/ml for one strain. Five percent of B. fragilis, 21% of other B. fragilis group species and 20% of Clostridium species (other than C. difficile, C. perfringens or C. ramosum) were resistant to clindamycin. No imipenem resistant isolates were found in this study. Gatifloxacin appears to have excellentin vitro activity against pulmonary isolates of anaerobes and very good activity against strains of the B. fragilis group.     

Antibiotic sensitivities of Neisseria gonorrhoeae in the Toronto area

The antibiotic sensitivity pattern of 3872 isolates of N. gonorrhoeae tested in Toronto from 1969 to 1973 is reviewed. An increase in resistance to both penicillin and tetracycline was noted up to 1971, but no further increase has occurred since then. Ninety-seven percent of 135 patients with “sensitive” strains (inhibited by 0.3 U/ml of penicillin and/or 0.5 μg/ml of tetracycline) were cured by either 8 g of tetracycline or 5,000,000 U of penicillin, whereas only 59% of 58 patients with “resistant” strains (requiring 1.0 U/ml of penicillin and/or 2.0 μg/ml of tetracycline for inhibition) were cured by the same dosage. Spectinomycin appears to be an acceptable alternative therapy. Maximum doses of the chosen drug are recommended in the hope of retarding further spread of more resistant organisms.     

The sodium, potassium and chloride of cerebral tissues: maintenance, change on stimulation and subsequent recovery

1. Cerebral tissues were prepared for incubation by cutting them from the brain rapidly and in situ, and the calcium concentration in the incubating medium was altered from the customary 2·8mm to 0·75mm. This provided incubated cerebral cortex with fluid and ion content more closely resembling that of the brain in vivo than hitherto obtained. 2. From a systematic difference in size between inulin spaces of slices with one and those with two cut surfaces, it was estimated that cutting directly affected a layer 0·02mm. thick. On the basis of the volume of this layer, it was calculated that the portion of the tissue not affected by cutting had an inulin space of 258 μl./g. initial wt., and during the process of preparation and incubation had gained 30μequiv. of sodium and 17 μequiv. of chloride/g. and had lost 14 μequiv. of potassium/g. 3. Several aspects of the ion content of the incubated tissue were compatible with the observed membrane potential of −60mv between cellular and extracellular phases. 4. In response to electrical stimulation, sodium of the non-inulin space increased from 28 to 57 μequiv./g., potassium decreased from 68 to 48 μequiv./g. and chloride increased from 16 to 22 μequiv./g. in the non-inulin space. These changes were complete in about 6min., and thereafter the concentrations remained steady during continued stimulation. Initial rates of change were 460 μequiv./g./hr. for sodium and 480 μequiv./g./hr. for potassium. 5. After stimulation was stopped the ionic composition of the tissue returned completely to its pre-stimulation state within 10min. Initial rates for extrusion of sodium and gain of potassium were 160 and 230 μequiv./g./hr. respectively.     

Gonococcal Antibiotic Resistance in Los Angeles

One hundred and thirty-five gonococcal isolates collected from Los Angeles in 1972 were studied for antibiotic susceptibility to penicillin, ampicillin, carbenicillin, tetracycline, minocycline, doxycycline and spectinomycin. Only 12 percent of the isolates were sensitive to 0.05 μg per ml of penicillin while 35 percent required at least 0.5 μg per ml for inhibition of growth. The results were slightly better with ampicillin and nearly the same with carbenicillin. Nineteen percent of the isolates required at least 1.0 μg per ml of tetracycline for inhibition of growth and the results were similar with either minocycline or doxycycline. Forty-nine percent were sensitive to 2.0 μg per ml spectinomycin, but 37 percent required at least 8.0 μg per ml for inhibition of growth.In this study nine of eleven isolates resistant to 1.0 μg per ml of tetracycline were also resistant to both penicillin and spectinomycin. Six came from endocervical sites of female patients who contributed only 37 percent of the total number of isolates studied.Correlation between the agar dilution and disc diffusion methods was satisfactory with penicillin but not with ether tetracycline or spectinomycin.