Adelaide Pesce

In vitro Activity of Rifaximin, Metronidazole and Vancomycin against Clostridium difficile and the Rate of Selection of Spontaneously Resistant Mutants against Representative Anaerobic and Aerobic Bacteria, Including Ammonia-Producing Species

Background: Rifaximin is a rifamycin derivative characterized by a wide antibacterial activity. This drug is neither absorbed by the gastrointestinal tract nor inactivated by gastric juices, and exerts its action entirely within the intestinal lumen. Methods: In this study, the activity of this antibiotic was compared with that of metronidazole and vancomycin against 93 Clostridium difficile isolates. The rate of emergence of bacteria spontaneously resistant to the new compound was also evaluated in relation to representative gram-positive and gram-negative strains. In terms of MIC50 values, rifaximin showed an intrinsic activity superior to that of the other agents. The emergence of spontaneously resistant strains was assessed with 46 aerobic (staphylococci, enterococci, Proteus spp., Citrobacter freundii, Providencia rettgeri, enteropathogenic, enteroinvasive, enterotoxigenic and entero- hemorrhagic Escherichia coli, and Salmonella enteritidis) and anaerobic (Clostridium spp., Bacteroides spp., Fusobacterium nucleatum and Peptococcus spp.) pathogens, most of them also ammonium producers. Two different methods, broth and agar dilution, were employed. Results: When liquid medium was employed, bacteria capable of sustained growth in 100 μg/ml of rifaximin were obtained after 2–5 transfers with gram-positive aerobic cocci, 2–3 transfers with gram-negative aerobic strains and 2–5 transfers with anaerobic species. At the highest dose used with the agar dilution method (8 × MIC), the frequency of emergence of spontaneously resistant mutants ranged from <1 × 10–9 to 1.6 × 10–8 with gram-positive aerobic and anaerobic cocci, while with aerobic and anaerobic gram-negative bacteria, this value ranged from <1 × 10–9 to 1.7 × 10–7. C. difficile showed a particularly low incidence of spontaneously resistant mutants (<1 × 10–9). The low incidence of resistant subpopulations selected by levels of 8 × MIC of rifaximin suggests that the high levels of the drug which were reached in the gastrointestinal lumen may further prevent the selection of mutants. Conclusion: The low toxicity, broad antibacterial activity and very poor absorption from the gastrointestinal tract of rifaximin suggest a potential therapeutic use for this drug in gastrointestinal diseases, as well as in the management of patients with cirrhosis and chronic portal-systemic encephalopathy.

Rifampicin-resistant RNA polymerase and NAD transferase activities in coliphage N4 virions

SEVERAL animal viruses carry within the infectious particle a number of enzymatic activities1,2. Prominent among these are the RNA synthetases of orthomixo, paramixo, rhabdo-and diplomaviruses3–6, the DNA-dependent RNA polymerase of poxviruses7 and the reverse transcriptases of oncornaviruses8,9, all of which play important roles in the virus life cycle. Exclusion of vital enzymes from tine virion of all complex bacterial viruses seems, on the other hand, to have been the result of the evolutionary trend followed by these agents. A notable exception to this rule seems to be represented by coliphage N4 (ref. 10). It has been reported that transcription of N4 early genes11,12 and incorporation of 32P-orthophosphate into several host proteins13 occur in Escherichia coli cells treated before infection with rifaimpicin and chloramphenicol. A possible explanation for these findings was that N4 injects into the cell the relevant enzymes along with its DNA. We report here the detection of a rifampicin-resistant RNA polymerase and a nicotinamide adenine dinucleotide (NAD) transferase in purified N4 particles.

Epidemiology of resistance to antimicrobial drugs in the major respiratory pathogens circulating in Europe

SummaryThere is an overwhelming consensus on the fact thatStreptococcus pneumoniae, Haemophilus influenzae andMoraxella catarrhalis represent the prevailing bacterial pathogens of community-acquired lower respiratory tract infections. Their specific incidence as causative agents of the more common syndromes is known to vary even profoundly depending on geographic location, and the same holds true for the rates of resistance to antimicrobial drugs. Europe does not escape the threat posed by the present pandemic spread of penicillin resistance inS. pneumoniae although, as expected, countries like Spain and France are greatly affected, while others including Germany, Italy, The Netherlands and the Scandinavian region are comparatively spared. In several sites multiple resistance has been described inS. pneumoniae and the most affected drugs include penicillin, the macrolides, co-trimoxazole and tetracycline. InH. influenzae synthesis of β-lactamases the main trait of resistance is expressed. Lack of susceptibility to β-lactams dictated by a different mechanism remains extremely rare. Considerable variations in the incidence of this characteristic are apparent when European countries are considered. France and Spain are again widely affected, while Germany, The Netherlands and Italy display rates of β-lactamase-positiveH. influenzae of about 10%.M. catarrhalis must be considered generally resistant to non-protected aminopenicillins since over 90% of these organisms produce β-lactamases.

In vitro assessment of the postantibiotic effect of lomefloxacin against gram-positive and gram-negative pathogens

We evaluated the postantibiotic effect (PAE) of lomefloxacin, a new difluorinated quinolone, on multiply drug-resistant gram-positive and gram-negative clinical pathogens. Lomefloxacin induced a PAE of about 2 hours in gram-negative and gram-positive bacteria exposed for 1 hour to concentrations corresponding to four times their minimum inhibitory concentrations (MIC). PAE values (hours) ranged from 1.8 with Serratia marcescens to 2.5 with Staphylococcus aureus. No significant differences in PAE values were found for ciprofloxacin or ofloxacin against the same pathogens. The PAE was increased when the time of drug exposure was increased to 2 hours. These results indicate that lomefloxacin has a PAE equivalent to that of other fluoroquinolones.

Parameters characterizing the in vitro activity of cefixime, a new oral broad spectrum cephalosporin, against respiratory and urinary pathogens

The wide and potent in vitro activity of cefixime, a new oral broad spectrum cephalosporin, has been confirmed on a collection of respiratory and urinary pathogens recently isolated in Italy. The new cephem emerged as the most bactericidal of all the comparators tested against several fast as well as slowly-growing gram-negative species including Enterobacteria, Haemophilus and Moraxella, irrespective of their ability to synthetize beta-lactamases. Among the gram-positive species Streptococcus pyogenes and S. pneumoniae were effectively covered. Cefixime activity was not adversely influenced by several important variables such as pH (over the range from 5 to 8), inoculum size (from 10(5) to 10(8) CFU per ml) and the presence of 50% human serum or urine. Time-kill tests confirmed a pronounced bactericidal potency of the drug especially towards common respiratory pathogens (H. influenzae, M. catarrhalis, S. pneumoniae and S. pyogenes). Killing of urinary strains was optimal at cefixime concentrations reached in urine since eradication, except for Proteus mirabilis, was enhanced with increasing levels of the drug. The absence of an untoward paradoxic effect on the rate of cefixime bactericidal action was confirmed by employing a dynamic bladder model simulating the pharmacokinetic parameters of the drug after a single 200 mg daily dosage. Interactions of cefixime with several other drugs that may be employed in combination therapy were generally prone to provide indifference and synergism while antagonism was never observed. Favorable interactions were also registered when cefixime acted with other antibiotics on partially resistant species such as Staphylococci and Pseudomonas. The new cephem seems to provide excellent opportunities for expanding oral cephalosporin therapy to a wide range of infections produced by susceptible pathogens in the adult and pediatric populations.

Antibacterial activity of ceftibuten, a new oral third generation cephalosporin.

Ceftibuten, a new oral third generation cephalosporin, was found to be the most active beta-lactam drug tested against members of the Enterobacteriaceae, inhibiting most strains at less than 4 micrograms/ml. All isolates of Branhamella catarrhalis, Haemophilus influenzae, and Neisseria spp. were highly susceptible to ceftibuten. Penicillin-sensitive pneumococci and pathogenic beta-hemolitic streptococci were also killed by ceftibuten. The antibacterial activity of this new drug, which results in rapid lysis of susceptible cells, was not significantly affected by serum, pH, inoculum size, media composition and growth conditions. Ceftibuten is characterized by a remarkable resistance to inactivation by most beta-lactamases synthetized by common gram-positive and gram-negative pathogens. The potent in vitro activity of ceftibuten in conjunction with its favorable pharmacokinetic profile render this new molecule an attractive candidate for the treatment of respiratory and urinary tract infections sustained by susceptible pathogens.

Stability in the Presence of Widespread β-Lactamases

Bacterial resistance to the beta-lactam drugs is extremely widespread, as a result of extensive drug use. Loss of susceptibility is primarily attributable to hydrolysis by inactivating enzymes, namely the beta-lactamases. While the number of characterised beta-lactamases may exceed 100, only a few are a problem in the treatment of community-acquired infections (TEM-1, TEM-2, SHV-1, BRO-1). Chromosomally mediated and extended-spectrum beta-lactamases are usually dominant in nosocomial pathogens where oral antibiotic therapy is seldom used. Therefore, the threat posed by beta-lactamases must be considered in general practice. Several effective strategies have been implemented in order to overcome beta-lactamase-mediated resistance, e.g. use of non-beta-lactam drugs or beta-lactamase inhibitors. Another option has been the development of new beta-lactam compounds that possess a high intrinsic stability against the hydrolytic action of common beta-lactamases. Among these compounds, the oral third generation cephalosporins represent an important breakthrough. Cefetamet pivoxil, a new oral third generation cephalosporin, is characterised by excellent antimicrobial potency against Enterobacteriaceae, and Moraxella (Branhamella) catarrhalis and Haemophilus influenzae, irrespective of their ability to produce beta-lactamases. The Gram-positive respiratory pathogens, Streptococcus pyogenes and penicillin-susceptible S. pneumoniae, are also satisfactorily covered. The activity of cefetamet has recently been corroborated in a survey conducted in Italy involving 4191 isolates. However, cefetamet shows no activity against enterococci, staphylococci, Listeria, alpha-streptococci, Pseudomonas, Acinetobacter and anaerobes. Given this antibacterial profile, cefetamet pivoxil may provide a useful alternative to other oral antibacterial agents in the empirical therapy of acute community-acquired respiratory and urinary tract infections. From the results of the Italian survey, cefetamet emerged as the only agent among those considered (which included cefuroxime, cefaclor, cefalexin, cefadroxil, ampicillin, amoxicillin/clavulanic acid, ampicillin/sulbactam, doxycycline, erythromycin and clindamycin) that might be selected as the drug of choice in the empirical therapy of outpatient infections.SummaryBacterial resistance to the β-lactam drugs is extremely widespread, as a result of extensive drug use. Loss of susceptibility is primarily attributable to hydrolysis by inactivating enzymes, namely the β-lactamases. While the number of characterised β-lactamases may exceed 100, only a few are a problem in the treatment of community-acquired infections (TEM-1, TEM-2, SHV-1, BRO-1). Chromosomally mediated and extended-spectrum β-lactamases are usually dominant in nosocomial pathogens where oral antibiotic therapy is seldom used.Therefore, the threat posed by β-lactamases must be considered in general practice. Several effective strategies have been implemented in order to overcome β-lactamase-mediated resistance, e.g. use of non-β-lactam drugs or β-lactamase inhibitors. Another option has been the development of new β-lactam compounds that possess a high intrinsic stability against the hydrolytic action of common β-lactamases. Among these compounds, the oral third generation cephalosporins represent an important breakthrough.Cefetamet pivoxil, a new oral third generation cephalosporin, is characterised by excellent antimicrobial potency against Enterobacteriaceae, and Moraxella (Branhamella) catarrhalis and Haemophilus influenzae, irrespective of their ability to produce β-lactamases. The Gram-positive respiratory pathogens, Streptococcus pyogenes and penicillin-susceptible S. pneumoniae, are also satisfactorily covered. The activity of cefetamet has recently been corroborated in a survey conducted in Italy involving 4191 isolates. However, cefetamet shows no activity against enterococci, staphylococci, Listeria, α-streptococci, Pseudomonas, Acinetobacter and anaerobes.Given this antibacterial profile, cefetamet pivoxil may provide a useful alternative to other oral antibacterial agents in the empirical therapy of acute community-acquired respiratory and urinary tract infections. From the results of the Italian survey, cefetamet emerged as the only agent among those considered (which included cefuroxime, cefaclor, cefalexin, cefadroxil, ampicillin, amoxicillin/clavulanic acid, ampicillin/sulbactam, doxycycline, erythromycin and clindamycin) that might be selected as the drug of choice in the empirical therapy of outpatient infections.

In vitro interactions between teicoplanin and other antibiotics against enterococci and staphylococci

In this study we have evaluated the in vitro activity of teicoplanin, a potent anti-staphylococcal and anti-enterococcal agent, in combination with rifampicin, netilmicin, amikacin, fosfomycin and imipenem. Drug interactions were assessed using a microtitre checkerboard dilution method and a time-kill test. With rifampicin, indifference or an additive effect was the prevalent outcome with both methods. With netilmicin and amikacin synergism was seen against half of the enterococcal strains, whereas indifference was more common with staphylococcal isolates. Synergism predominated when teicoplanin was combined with fosfomycin and imipenem. Under no circumstances and with no combination of drugs were we able to detect antagonistic effects.

Bactericidal Activity, Morphological Alterations, and Synergistic Interactions of Rufloxacin, a New Fluoroquinolone, Alone and in Combination with Its N-Desmethylate D Derivative (MF922)

The in vitro activity of rufloxacin, alone and in combination with its metabolite (MF 922) against common respiratory and urinary tract pathogens and anaerobes was assessed. No synergistic interaction between rufloxacin and MF 922 was observed by the checkerboard technique against aerobic bacteria. When the time-kill system was employed, 24 synergistic interactions were noted out of 30 tests performed (80%), of which 14 (100%) were with Enterobacteriaceae isolates, 2 with Moraxella catarrhalis (50%), 4 with Haemophilus influenzae (100%), 2 with Staphylococcus aureus (50%) and 2 with Streptococcus pneumoniae and Streptococcus pyogenes (50%). Synergism was found with all Bacteroides fragilis irrespectively of the method used. Rufloxacin alone or in combination with MF 922, at concentrations achieved in vivo, induced morphological alterations in all the pathogens analyzed with the exception of M. catarrhalis and H. influenzae. Towards S. pyogenes and S. pneumoniae, the same levels of rufloxacin and MF 922 were capable of inducing only bacteriostatic rather than bactericidal effect, followed by reversible morphological modifications. The presence of 50% human serum in the test media did not affect the results.

Dactimicin, a new aminoglycoside: In vitro activity, post-antibiotic effect and interaction with other antibiotics

The in vitro activity of the new aminoglycoside dactimicin in comparison to amikacin was tested alone and in combination with piperacillin, mezlocillin and ceftazidime against freshly isolated clinical pathogens. Dactimicin was more active than amikacin againstEnterobacter cloacae, Providencia rettgeri and Salmonella spp., and less active than amikacin againstEscherichia coli, Pseudomonas aeruginosa andAcinetobacter anitratus. Using the checkerboard technique, the combination of either dactimicin or amikacin with the other drugs was shown to result in synergistic interaction against most of the 23 strains tested. Dactimicin-ceftazidime and amikacin-ceftazidime were the most effective combinations, demonstrating synergism against 91 % and 95 % of the isolates respectively. Antagonism was not encountered. Using the time-kill method, synergism was seen in most cases, indifference rarely being seen; antagonism was not observed. Dactimicin induced a post-antibiotic effect which ranged from 1 h forEnterobacter cloacae to 2.4 h forEscherichia coli. An average post-antibiotic effect of 0.6 h was also seen when dactimicin was combined with piperacillin, mezlocillin and ceftazidime. The findings indicate that dactimicin compares favorably in vitro with amikacin and suggest that clinical trials with this drug alone or in combination are warranted.