Search

The present study demonstrates the in vitro effectiveness of the macrolide rokitamycin and the phenothiazine compound chlorpromazine against Acanthamoeba castellanii . Growth curve evaluations revealed that both drugs inhibit trophozoite growth in dose- and time-dependent ways. The effects of both drugs when they were used at the MICs at which 100% of isolates are inhibited were amoebistatic, but at higher doses they were amoebicidal as well as cysticidal. Experiments showed that when rokitamycin was associated with chlorpromazine or amphotericin B, rokitamycin enhanced their activities. Furthermore, low doses of rokitamycin and chlorpromazine, alone or in combination, blocked the cytopathic effect of A. castellanii against WKD cells derived from the human cornea. These results may have important significance in the development of new anti- Acanthamoeba compounds.

In Spain, the frequency of resistance of group A streptococci (GAS) to erythromycin was very low, increasing from 0 to 0.7% in 1989 and 1990 (1, 7) to approximately 3% in 1991 to 1994 (2). The purpose of the present study was to determine the susceptibilities of recent isolates of GAS to macrolides and, in the case of resistant isolates, to ascertain their patterns of erythromycin resistance. We also studied the in vitro activities of penicillin G and clindamycin. A total of 222 isolates of GAS were collected from April to August 1996 from outpatient samples in Madrid, Spain. A total of 203 isolates were throat swab samples and 19 were from other sources. Identification was done by biochemical tests according to standard criteria (8). Penicillin G, erythromycin, azithromycin, miocamycin, and clindamycin were tested. The MICs were determined by the agar dilution method with Mueller-Hinton agar supplemented with 5% defibrinated horse blood. The plates were incubated overnight at 35°C in a 5% carbon dioxide atmosphere. The procedures for susceptibility testing were those recommended by the National Committee for Clinical Laboratory Standards (6). The MIC breakpoint for miocamycin resistance was >4 μg/ml, as defined by the Comite de l’Antibiogramme de la Societe Francaise de Microbiologie (10). Erythromycin resistance was classified according to the results of disk diffusion, with a double-disk test with erythromycin and clindamycin disks, as previously described (9), to test for inducible resistance. The MIC ranges and the MICs at which 50 and 90% of isolates were inhibited (MIC50s and MIC90s, respectively) for the 222 isolates of Streptococcus pyogenes are given in Table ​Table1.1. Penicillin remained exquisitely active, with an MIC90 of 0.03 μg/ml. TABLE 1 In vitro susceptibilities of 222 recent GAS strains to five antimicrobial agents Thirty-nine (17.6%) of the isolates tested were resistant to erythromycin (MIC breakpoint, ≥1 μg/ml). In contrast, only 0.5% of the S. pyogenes isolates were resistant to clindamycin (MIC90, ≤0.12 μg/ml). The resistance to both 14- and 15-membered macrolides tested was 17.6%, whereas the resistance to miocamycin, a 16-membered macrolide, was 0.5%. Thirty-seven (95%) of 39 erythromycin-resistant GAS were susceptible to clindamycin (MICs, ≤0.12 μg/ml) and miocamycin (MICs, 0.5 to 1 μg/ml). One isolate was resistant to erythromycin (MIC, >64 μg/ml), azithromycin (MIC, >16 μg/ml), miocamycin (MIC, >64 μg/ml), and clindamycin (MIC, >64 μg/ml), which indicates cross-resistance to macrolides, lincosamides, and streptogramin B-type antibiotics (MLSB phenotype). Another erythromycin-resistant strain was susceptible to clindamycin but showed an inducible type of erythromycin resistance. All erythromycin-resistant isolates were also resistant to the 15-membered macrolide azithromycin. However, the 16-membered macrolide miocamycin showed good activity against these isolates. The MICs for 37 of 39 erythromycin-resistant isolates were moderate (8 to 32 μg/ml), but the MICs of clindamycin were the same as those for the erythromycin-susceptible isolates (≤0.12 μg/ml); this pattern has been described with the M phenotype (11) and “novel resistance” (9). With this phenotype, susceptibility to clindamycin and streptogramin B antibiotics but resistance to 14- and 15-membered macrolides is seen (11). The mechanism of erythromycin resistance involves active efflux. The rate of isolation of strains with this novel phenotype among the erythromycin-resistant GAS of our work is even higher than that recently reported in the United States (75%) (11), although the incidence of erythromycin-resistant GAS is rare in this country (3). However, 50% of erythromycin-resistant GAS isolated in Japan (5) and in Taiwan (4) were also resistant to clindamycin and tetracycline. In conclusion, our report documents the emergence of high rates of resistance to erythromycin in S. pyogenes in Spain. Strains with the M phenotype account for the majority of those isolates, which in a sample of this size represents a significant finding. Clindamycin remained active against these strains and might be useful for treatment of patients with clinical failure after phenoxymethyl penicillin treatment and for penicillin-allergic patients. We suggest that clinical laboratories review their local susceptibility data. If erythromycin resistance rates are high, we recommend that susceptibilities to both macrolides and clindamycin in S. pyogenes isolates be routinely tested.

The susceptibilities of 100 clinical isolates of Campylobacter jejuni to rokitamycin, a new macrolide, and eight other oral antibiotics were tested by using a broth microdilution method. Rokitamycin demonstrated antibacterial activity that was higher than those of fosfomycin and the tetracyclines and comparable to those of erythromycin and clindamycin, whereas it was slightly less active than the quinolones.

A radioimmunoassay system has been developed for the measurement of two major metabolites of 9,3"-diacetylmidecamycin, Mb-6 and Mb-12. A radioimmunoassay for Mb-6 was performed by using anti-Mb-6 serum and a [125I]tyramined Mb-6 derivative as a radiolabeled antigen. The labeled antigen was prepared by the chloramine T method. The antiserum was obtained from a rabbit immunized with Mb-6 conjugated to bovine serum albumin. The obtained antiserum was cross-reactive with two other metabolites of 9,3"-diacetylmidecamycin, Mb-2 and Mb-12, in addition to Mb-6. This Mb-6 radioimmunoassay system could detect Mb-6 concentrations as low as 100 pg/ml of serum. The coefficients of variation were 4.5% (intra-assay) and 5.1% (inter-assay). A radioimmunoassay for Mb-12, using anti-midecamycin serum and a [125I]tyramined-Mb-12 derivative, has also been developed. The antiserum was cross-reactive only with Mb-12 among the 9,3"-diacetylmidecamycin metabolites. This Mb-12 radioimmunoassay system could detect Mb-12 concentrations as low as 2 ng/ml. The intra- and inter-assay variances were 5.9 and 5.8%, respectively.

Fifty strains of Mycoplasma pneumoniae in L cells were tested for susceptibility to macrolide antibiotics. Rokitamycin, a new macrolide antibiotic, was most active against these organisms, with an MIC for 90% of strains of 0.007 microgram/ml. The MICs of erythromycin, josamycin, and kitasamycin for 90% of strains were 0.03, 0.03, and greater than or equal to 0.06 microgram/ml, respectively. Based on these results, rokitamycin is a promising antibiotic for the treatment of mycoplasmal infections, and further clinical investigations are needed.

The accumulation of macrolide antibiotics in Bacteroides fragilis ATCC 25285 was increased in the order erythromycin, josamycin, and rokitamycin, depending on hydrophobicity. The half-times of efflux were also prolonged in the same order. Furthermore, MICs of the antibiotics were correlated with the extent of hydrophobicity. These findings suggest that the macrolide antibiotics are accumulated in B. fragilis by means of their hydrophobic properties, and the efficient accumulation of the drugs may explain the susceptibility of this gram-negative bacterium to macrolides.