Search PubMed⌕ Search

Biomedical subjects

M I Morosini

Publications and source records attributed to M I Morosini.

18 recordsLinked to original sources
PubMed2001

In vitro activity of telithromycin against Spanish Streptococcus pneumoniae isolates with characterized macrolide resistance mechanisms.

The susceptibilities to telithromycin of 203 Streptococcus pneumoniae isolates prospectively collected during 1999 and 2000 from 14 different geographical areas in Spain were tested and compared with those to erythromycin A, clindamycin, quinupristin-dalfopristin, penicillin G, cefotaxime, and levofloxacin. Telithromycin was active against 98.9% of isolates (MICs, < or =0.5 microg/ml), with MICs at which 90% of isolates are inhibited being 0.06 microg/ml, irrespective of the resistance genotype. The corresponding values for erythromycin were 61.0% (MICs, < or =0.25 microg/ml) and >64 microg/ml. The erm(B) gene (macrolide-lincosamide-streptogramin B resistance phenotype) was detected in 36.4% (n = 74) of the isolates, which corresponded to 93.6% of erythromycin-intermediate and -resistant isolates, whereas the mef(A) gene (M phenotype [resistance to erythromycin and susceptibility to clindamycin and spiramycin without blunting]) was present in only 2.4% (n = 5) of the isolates. One of the latter isolates also carried erm(B). Interestingly, in one isolate for which the erythromycin MIC was 2 microg/ml, none of these resistance genes could be detected. Erythromycin MICs for S. pneumoniae erm(B)-positive isolates were higher (range, 0.5 to >64 microg/ml) than those for erm(B)- and mef(A)-negative isolates (range, 0.008 to 2 microg/ml). The corresponding values for telithromycin were lower for both groups, with ranges of 0.004 to 1 and 0.002 to 0.06 microg/ml, respectively. The erythromycin MIC was high for a large number of erm(B)-positive isolates, but the telithromycin MIC was low for these isolates. These results indicate the potential usefulness of telithromycin for the treatment of infections caused by erythromycin-susceptible and -resistant S. pneumoniae isolates when macrolides are indicated.

Anti-Bacterial Agents↗
PubMed2000

Biological cost of AmpC production for Salmonella enterica serotype Typhimurium.

Chromosomally mediated AmpC-type beta-lactamases are frequently found among Enterobacteriaceae. Hyperproduction of AmpC beta-lactamase results in high-level resistance to beta-lactam antibiotics. One striking feature of Salmonella is the absence of the structural ampC gene, encoding AmpC beta-lactamase, in contrast with other members in the Enterobacteriaceae family, such as Escherichia, Citrobacter, or Enterobacter. The horizontal acquisition of ampC genes is one of the causes of the increased resistance to extended-spectrum cephalosporins and beta-lactamase inhibitors among gram-negative rods. Nevertheless, despite the high number of beta-lactam-resistant Salmonella isolates so far described, only two strains expressing resistance to cephalosporin and beta-lactamase inhibitors which is mediated by AmpC-type enzymes have been found. In this work, data are provided which support the possibility that the maintenance and expression of the ampC gene may represent an unbearable cost for Salmonella in terms of reduction of some of its lifestyle attributes, such as growth rate and invasiveness. The deleterious AmpC burden can be eliminated by decreasing the production of AmpC when both the regulatory gene, ampR, and ampC are present in Salmonella. Thus, it is suggested that the two genes have to be acquired together by Salmonella, leading to an inducible beta-lactam resistance phenotype. AmpC synthesis did not produce major variations in the peptidoglycan composition of Salmonella.

Animals↗
PubMed2000

Selection of naturally occurring extended-spectrum TEM beta-lactamase variants by fluctuating beta-lactam pressure.

Despite the large number of in vitro mutations that increase resistance to extended-spectrum cephalosporins in TEM-type beta-lactamases, only a small number occur in naturally occurring enzymes. In nature, and particularly in the hospital, bacteria that contain beta-lactamases encounter simultaneous or consecutive selective pressure with different beta-lactam molecules. All variants obtained by submitting an Escherichia coli strain that contains a bla(TEM-1) gene to fluctuating challenge with both ceftazidime and amoxicillin contained only mutations previously detected in naturally occurring beta-lactamases. Nevertheless, some variants obtained by ceftazidime challenge alone contained mutations never detected in naturally occurring TEM beta-lactamases, suggesting that extended-spectrum TEM variants in hospital isolates result from fluctuating selective pressure with several beta-lactams rather than selection with a single antibiotic.

Anti-Bacterial Agents↗
PubMed1999

The complexed structure and antimicrobial activity of a non-beta-lactam inhibitor of AmpC beta-lactamase.

Beta-lactamases are the major resistance mechanism to beta-lactam antibiotics and pose a growing threat to public health. Recently, bacteria have become resistant to beta-lactamase inhibitors, making this problem pressing. In an effort to overcome this resistance, non-beta-lactam inhibitors of beta-lactamases were investigated for complementarity to the structure of AmpC beta-lactamase from Escherichia coli. This led to the discovery of an inhibitor, benzo(b)thiophene-2-boronic acid (BZBTH2B), which inhibited AmpC with a Ki of 27 nM. This inhibitor is chemically dissimilar to beta-lactams, raising the question of what specific interactions are responsible for its activity. To answer this question, the X-ray crystallographic structure of BZBTH2B in complex with AmpC was determined to 2.25 A resolution. The structure reveals several unexpected interactions. The inhibitor appears to complement the conserved, R1-amide binding region of AmpC, despite lacking an amide group. Interactions between one of the boronic acid oxygen atoms, Tyr150, and an ordered water molecule suggest a mechanism for acid/base catalysis and a direction for hydrolytic attack in the enzyme catalyzed reaction. To investigate how a non-beta-lactam inhibitor would perform against resistant bacteria, BZBTH2B was tested in antimicrobial assays. BZBTH2B significantly potentiated the activity of a third-generation cephalosporin against AmpC-producing resistant bacteria. This inhibitor was unaffected by two common resistance mechanisms that often arise against beta-lactams in conjunction with beta-lactamases. Porin channel mutations did not decrease the efficacy of BZBTH2B against cells expressing AmpC. Also, this inhibitor did not induce expression of AmpC, a problem with many beta-lactams. The structure of the BZBTH2B/AmpC complex provides a starting point for the structure-based elaboration of this class of non-beta-lactam inhibitors.

Anti-Bacterial Agents↗
PubMed1998

An extended-spectrum AmpC-type beta-lactamase obtained by in vitro antibiotic selection.

A predictive approach was assayed to evaluate the possibility of mutant Amp-C beta-lactamase emergence with increased substrate spectrum (including new C-3' quaternary ammonium cephems). The ampC gene encoding the AmpC beta-lactamase from Enterobacter cloacae was cloned and expressed in an AmpC-defective strain of E. coli. After the AmpC containing strain was challenged with cefpirome, an ampC variant encoding an enzyme with increased resistance to cefpirome and cefepime was selected. In addition, this variant conferred increased resistance to penicillins and third generation cephalosporins. The complete nucleotide sequence of the gene was determined. The deduced peptide sequence showed a single change with respect to the wild-type gene: valine to glutamic acid at position 318 of the native protein (298 of the mature enzyme). The potential emergence and spread of this type of AmpC variants among pathogens should be considered.

Amino Acid Sequence↗
PubMed1998

Antibiotic-selective environments.

The evolution and spread of antibiotic resistance depends on the antibiotic pressure exerted in the microbial environment. Selective effects occur in selective compartments, where particular antibiotic concentrations result in a differential growth rate of resistant bacterial variants. This may happen even at very low antibiotic concentrations able to select low-level-resistant bacteria. When more than one antibiotic is present in the environment, the multiple and fluctuating pressure produces the selection of bacterial variants that use multiple or multipurpose mechanisms or optimize a single mechanism of resistance to survive under the variable environmental conditions. Host factors such as immunity contribute to the selective process. Antibiotics themselves may promote bacterial diversity, either mediated by the random drift effect or triggering the increase of mutational events under bacterial stress. Analysis of selective environment-related antibiotic-host-bacteria interactions is essential to understanding the biology of antibiotic resistance.

Adaptation, Physiological↗
PubMed1998

A237T as a modulating mutation in naturally occurring extended-spectrum TEM-type beta-lactamases.

A TEM-1 beta-lactamase derivative containing the single amino acid substitution A237T slightly increased (from 24 to 32 microg/ml) the cephalothin MIC for Escherichia coli RYC1000 but did not influence the activities of cefotaxime, ceftazidime, and aztreonam (MICs of 0.03, 0.12, and 0.06 microg/ml, respectively). Despite its apparent neutrality, addition of the A237T mutation to the pair of mutations characterizing TEM-10 (R164S and E240K) had a strong effect on substrate preference. Ceftazidime and aztreonam MICs decreased from 128 and 16 microg/ml to 16 and 2 microg/ml, respectively. In contrast, the cefotaxime MIC increased from 0.5 to 4 microg/ml. The acquisition of apparently neutral or even deleterious mutations results in a very effective mechanism of resistance to different beta-lactams that may be simultaneously or subsequently present in the environment. We propose here that the mutation in position 237 is an example of a modulating mutation and that consideration of this type of mutation may be important for understanding the evolution of beta-lactamases.

Escherichia coli↗
PubMed1998

Selection of very small differences in bacterial evolution.

As the Science of Biology is constantly changing due to new discoveries and advanced techniques it is essential that a systematic study of the environmental causes of natural selection on microorganisms be conducted. Very small phenotypic differences among individuals within bacterial populations arise as a result of spontaneous genetic variation, but the evolutionary importance of these small changes is frequently considered to be non-significant. Recent in vitro experiments indicate that efficient selection of these very small differences may take place in environmental compartments where a particular intensity of the selective agent is exerted. Model studies based on competition between bacterial populations only differing in one or two amino acid changes of a detoxifying antibiotic enzyme (e.g. beta-lactamase) have shown that at a narrow range of antibiotic concentrations the variant population is strongly selected over the original type, despite the extremely low phenotypic differences in antibiotic susceptibility. These selective concentrations are expected to occur in precise environmental compartments (selective compartments). Due to the high frequency of structured habitats in natural environments, the intensity of selective agents is commonly exerted along certain gradients. Each one of the points forming these gradients (or intersection among gradients) may have a particular selective ability for a specific genetic variant. Considering the environment as a composition of an extremely high number of specific selective compartments may help to understand the existence of high levels of genetic variability in natural bacterial populations. This may be one of the clues towards the unraveling of bacterial evolution.

Bacteria↗
PubMed1997

The antibiotic selective process: concentration-specific amplification of low-level resistant populations.

The biochemistry and genetics of antibiotic resistance are far better known than the equally important events underlying the selection of resistant populations. The hidden selection of low-level resistant variants may be a key process in the emergence of high-level antibiotic resistance. Different low-level resistant bacterial subpopulations may be specifically selected by different low antibiotic concentrations. The space in the environment (human body) where a given selective concentration exists represents the selective compartment. For pharmacokinetic reasons, low antibiotic concentrations occur in a larger selective compartment and persist longer than high antibiotic concentrations. The specific selection of low-level variants by low concentrations of antibiotic can be reproduced in experimental in vitro models using mixtures of susceptible and low-level resistant populations. We demonstrated this in Escherichia coli strains harbouring TEM-1, TEM-12 and TEM-10 beta-lactamases challenged by cefotaxime, and also Streptococcus pneumoniae strains with various levels of penicillin resistance challenged by amoxicillin or cefotaxime. In both cases, four hours of antibiotic challenge produced selective peaks of low-level resistant variant populations at low-level antibiotic concentrations. We conclude that variants with small decreases in antibiotic susceptibility may be fully selectable under in vivo circumstances; on the other hand, low-level antibiotic concentrations may have a considerable selective effect on the emergence of antibiotic resistance.

Anti-Bacterial Agents↗
PubMed1997

[Meropenem against bacteria carriers of wide spectrum TEM beta-lactamases: evolutive aspects].

The so-called wide spectrum beta-lactamases (WSBLs) are able to hydrolyze wide spectrum cephalosporins or monobactamics such as cefotaxime, ceftriaxone, ceftazidime, cefepime, cefpiroma or aztreonam. The natural wide spectrum beta-lactamases are mutational variants of TEM-1 consisting in the substitution of one of more amino acids within seven well defined positions in the molecule. Given the expected extremely low frequency for the simultaneous production of double or triple mutations, it is plausible that one of the mutational changes has been independently selected. A plurimutational remodelling of the TEM-beta-lactamase molecule is successively produced with the consequent appearance of highly effective ESBLs. Mutagenesis techniques allow clean molecular variants to be produced and allow the mutational effects under homogeneous conditions of bacterial strain, the plasmid implicated or the genic promotor to be studied. Meropenem remains active versus all the wide spectrum beta-lactamases referred in the 2be group of Bush, Jacoby and Medeiros as well as the new beta-lactamases produced in vitro by directed mutagenesis.

Escherichia coli↗
PubMed1996

Characterization of a nosocomial outbreak involving an epidemic plasmid encoding for TEM-27 in Salmonella enterica subspecies enterica serotype Othmarschen.

A ceftazidime-resistant Salmonella enterica subspecies enterica serotype Othmarschen strain, harboring the plasmid-mediated new extended-spectrum beta-lactamase TEM-27, was involved in a nosocomial outbreak (8 patients) at the Pediatric Cardiology Department of the Ramón y Cajal Hospital in Madrid. Genomic DNA polymorphism analysis, using an arbitrarily primed polymerase chain reaction, demonstrated the clonal nature of all Salmonella isolates. The plasmid encoding TEM-27 (pJMM1) was characterized by EcoRI, SacI, and BglI restriction. An identical restriction pattern was found in plasmid from all S. enterica strains. Possible in vivo intergeneric plasmid spread was suggested by the identification of an identical plasmid in Escherichia coli RYC5H and Enterobacter cloacae RYC39737 strains isolated during the outbreak. Results indicate that this outbreak involved dissemination of a single ceftazidime-resistant Salmonella strain and the spread of a single TEM-27-encoding plasmid among different Enterobacteriaceae.

Child, Preschool↗
PubMed1995

Single amino acid replacements at positions altered in naturally occurring extended-spectrum TEM beta-lactamases.

By directed mutagenesis, we constructed a set of seven TEM-1 derivatives containing single replacements in each one of the amino acids substituted in naturally occurring extended-spectrum TEM beta-lactamases. The exact contribution of each mutation to the resistance phenotype was determined. In addition, mutant enzyme production and stabilities were studied. Five of seven mutations determined to some extent variations in cephalosporin and/or monobactam activity. Dramatic changes in the hydrolysis of ceftazidime and aztreonam occurred when a serine was at position 164. Changes at positions 104, 238, and 240 showed more leaky variation in activity towards cephalosporins and aztreonam. Replacements at positions 237 and 265 caused no variation in susceptibility to cephalosporins. Interestingly, the change from Gln to Lys at position 39 found in TEM-2, classically considered a neutral change, slightly but consistently increased the MIC of ceftazidime and aztreonam. The in vitro construction of mutations appearing in naturally occurring TEM-beta-lactamases, studied in the same genetic context, may help to understand the evolution of extended-spectrum beta-lactamases.

Amino Acids↗
PubMed1995

New extended-spectrum TEM-type beta-lactamase from Salmonella enterica subsp. enterica isolated in a nosocomial outbreak.

A new extended-spectrum beta-lactamase was detected in a lactose-positive Salmonella enterica subsp. enterica strain that caused a nosocomial outbreak involving eight patients in a pediatric cardiology unit. This strain showed high levels of resistance to ceftazidime and aztreonam and relatively low levels of resistance to cefotaxime and ceftriaxone. Resistance was associated with a conjugative plasmid of 59 kb, which encoded a new beta-lactamase with an isoelectric point of 5.9 that strongly hydrolyzed ceftazidime and to a much lesser extent hydrolyzed cefotaxime. The enzyme activity was inhibited by clavulanate. The corresponding bla gene was cloned and sequenced. The deduced amino acid sequence showed three significant amino acid replacements with respect to the TEM-1 sequence: Arg-164-->His, Glu-240-->Lys, and Thr-265-->Met. This combination is unique among extended-spectrum beta-lactamases and served to characterize the new enzyme, TEM-27.

Amino Acid Sequence↗
PubMed1994

In vitro selective antibiotic concentrations of beta-lactams for penicillin-resistant Streptococcus pneumoniae populations.

Therapeutic regimens containing beta-lactam antibiotics are selecting penicillin-resistant Streptococcus pneumoniae populations all over the world. The selective pressure after 4 h of exposure to different concentrations of amoxicillin, cefixime, cefuroxime, and cefotaxime for low-level or high-level penicillin-resistant S. pneumoniae was evaluated in an in vitro model with mixed populations with penicillin susceptibilities of 0.015, 0.5, 1, and 2 micrograms/ml. The antibiotic concentration selecting for low-level resistance strongly reduced the susceptible population. Increasing antibiotic concentrations tended to decrease the total proportion of penicillin-resistant bacteria because of reduced numbers of the low-level-resistant population. The antibiotic concentration selecting for high-level resistance produced fewer resistant populations, but most of the organisms selected represented high-level resistance. In general, amoxicillin was a good selector for the low-level-resistant population and a poor selector for high-level resistance; cefuroxime and cefotaxime were poor selectors for low-level resistance and better selectors than amoxicillin for high-level penicillin resistance. Cefixime was the best selector of low-level penicillin resistance. When only resistant populations were mixed, the strains with high-level resistance were selected even at low antibiotic concentrations. Determination of the effects of selective antibiotic concentrations on mixed cultures of bacteria expressing different antibiotic resistance levels may help researchers to understand the ecology and epidemiology of penicillin-resistant S. pneumoniae populations.

Amoxicillin↗
PubMedDate not supplied

In vitro activity of cefpirome compared with other third generation cephalosporins against nosocomial isolates in Argentina.

The in vitro activity of cefpirome was evaluated against strains that showed conflicting results for third generation cephalosporins. Against isolates with derepressed inducible chromosomal cephalosporinase (n = 40) cefpirome was the sole cephalosporin with an MIC90 in the susceptible range; Klebsiella spp. with plasmid-mediated beta-lactamases (broad spectrum SHV-2 or SHV-2 type) (n = 40) remained most susceptible to ceftizoxime and cefpirome; against aminoglycoside-resistant Pseudomonas aeruginosa (n = 50), cefpirome was as active as ceftazidime and cefoperazone; against oxacillin-susceptible and oxacillin-resistant Staphylococcus spp., (n = 40), cefpirome was more active than other third generation cephalosporins but killing was inadequate against both oxacillin-resistant staphylococci and enterococci.

Argentina↗