Subtelomeric secondary metabolite (SM) gene clusters are frequently surrounded by DNA repeats and transposon-like elements. The A. nidulans penicillin cluster, 30 kb from the telomere of chromosome VI, is bordered by such elements. Deletions of penicillin telomere proximal and distal border regions resulted in decreased penicillin production. A 3.7 kb distal region called PbIa, consisting of the putative transposable element DNA-2, was examined further where its replacement by a pyrG marker presented a similar phenotype as loss of the global SM regulator LaeA, resulting in a decrease in penicillin gene expression and product formation. In contrast, placement of the pyrG marker on either side of PbIa had no effect on penicillin synthesis. A requirement for PbIa in penicillin production was also apparent in a histone deacetylase mutant, ΔhdaA, enhanced for penicillin production. Trans complementation of the PbIa element near and within the terrequinone A cluster on chromosome V did not restore penicillin biosynthesis or increase production of terrequinone A. Taken together, this data provides evidence for transposon involvement in SM cluster regulation.

In vitro studies with penicillin and [3lHlstreptomycin in four strains of streptococci (S. faecalis, S. sanguis, and S. mitis) were performed by simultaneously measuring the rates of bacterial killing and uptake of streptomycin. In S. faecalis, penicillin stimulated streptomycin uptake, as has been shown by Moellering and Weinberg (R. C. Moellering, Jr., and A. N. Weinberg, J. Clin. Invest. 50:2580-2584). Moreover, the antibiotic combination was associated with an enhanced bactericidal rate which temporally correlated with ,-lactam-induced aminoglycoside uptake. In contrast, in viridans group streptococci (S. sanguis and S. mitis) penicillin had no effect on streptomycin uptake and a minimal effect on bactericidal rate when compared with either drug alone. These data suggested that the stimulation of streptomycin uptake in streptococci by penicillin is strain or species specific and that important differences exist between enterococci and viridans group streptococci regarding the mechanisms of ,-lactam-aminoglycoside potentiation.Antibiotic synergy is perhaps best exemplified by the use of a P-lactam combined with an aminoglycoside in the therapy of enterococcal endocarditis. The enhanced antibacterial activity of the combination has been well established both in clinical trials (26) and in an experimental model of endocarditis (14). In studies on the mechanism of synergy in enterococci, Moellering and colleagues (35, 36) demonstrated that penicillin enhanced intracellular entry of otherwise sublethal concentrations of streptomycin, suggesting that the mechanism of sytergy in Streptococcusfaecalis was similar to that shown earlier in Escherichia coli by Plotz and Davis (36). However, from a mechanistic standpoint it is not clear how penicillin would enhance uptake of aminoglycosides in most gram-positive bacteria since no barrier to aminoglycoside penetration comparable to that of gramnegative bacilli has been described. Time kill studies (35,36) also demonstrated that enhancement of streptomycin entry was associated with an increase in the rate of killing during 24 h as determined by the inability of cells to form colonies on agar after overnight incubation. Twenty-four-hour time kill studies have since become a generally accepted in vitro method for examining bactericidal synergism in both grampositive and gram-negative bacteria.Numerous 24-h in vitro time kill studies have shown apparent synergy (i.e., a >2 logl0 enhanced bactericidal effect) between P-lactams and aminoglycosides in grampositive organisms (5,12,20,21,41,44). In vitro (22,24, 33) and in vivo (23, 25, 33) studies with Staphylococcus aureus and Staphylococcus epidermidis in our laboratory, however, indicated that a second mechanism by which cell wall-active antibiotics (,-lactams, vancomycin) enhance the bactericidal effect of aminoglycosides (or rifampin) is by preventing regrowth of relatively resistant subpopulations. Twentyfour-hour time kill studies often do not distinguish between the aforementioned mechanisms of drug potentiation, and ...

Staphylococcus aureus toxins represent a public health challenge all over the world. This study aims to analyze the prevalence of genes encoding the staphylococcal leukocidins and their correlation with antimicrobial susceptibility and the source of isolation. For this purpose, the susceptibility of 75 S. aureus isolates to 12 antimicrobial agents was investigated. The leukocidins genes (lukD, lukE, lukF and lukS) were detected by polymerase chain reaction (PCR). The ability to express these genes was assessed among 20 isolates by RT-PCR. The most prevalent luk genes were lukF gene (73.3%), followed by lukE (64%), lukD (44%) and lukS (34.7%). Expression of lukD, lukE and lukS genes were variable. lukF gene was not expressed by any of the tested isolates. A statistically significant association was found between lukF occurrence and burn isolates. Besides that, lukF gene was more prevalent among amoxicillin-clavulenic acid and amikacin resistant isolates, while lukE was predominant with gentamicin resistant isolates. High expression level of lukD was found in MRSA and MDR isolates.

The aim of the present investigation is to trace the epidemiological relatedness between different isolates of proteus sp. obtained from Dakahleia governorate as compared with similar sources from the adjacent Damietta governorate in Egypt. A total of 500 bacterial isolates were collected from human and animal sources including urine samples, sputum, wound, throat and ear swabs, endotracheal aspirate, breast swab, bile aspirate, blood samples, human and animal stools. From a total of 70 Proteus isolates, 62 were identified as Proteus mirabilis while eight as Proteus vulgaris. All Proteus mirabilis isolates were screened for susceptibility to ten different antimicrobials and were classified into 29 different patterns. Two resistant strains were found to belong to pattern No. 10 (isolates No. 11 and 19). A second pair of isolates namely No. 27&31 was classified into pattern No. 18. A third pair of (isolates No 30 and 16) was found to have great similarity to pattern No. 18 with a minor difference in resistance against one or two antibiotics. For a molecular characterization of the resistance determinants, 15 of the highly resistant Proteus mirabilis isolates were screened for the presence of different classes of integrons. Class 1 was the dominantly detected type that confers resistance to trimethoprim and aminoglycosides. One isolate was found to contain class 2. Similar integron components (dfrA15) could be proved among strains of different human sources (No. 11 & 19). Another similar type of intergon components (dfrA17)-(aadA5) was found in human isolates 31, 27, 30 & 16. In conclusion, three pairs of the studied isolates have been turned out to be epidemiologically related with the possibility of hospital infection and of strain transfer from one governorate to the other.