Comparison of the in-vitro activities of ceftazidime and new cephalosporins against 107 strains of Pseudomonas aeruginosa and 249 strains of cefazolin-resistant Enterobacteriaceae

Three hundred and fifty-six bacterial strains, including 107 strains of Pseudomonas aeruginosa, were tested with ceftazidime and five other agents. All bacterial strains were resistant to cefazolin but were sensitive to 32 mg/l of ceftazidime or less.

Antibiotic susceptibility and mechanisms of beta-lactam resistance in 1310 strains of pseudomonas aeruginosa: a French multicentre study (1996)

A total of 1310 consecutive strains of Pseudomonas aeruginosa were collected in 11 French hospitals in 1996. The percentages of susceptible isolates measured by the agar dilution method were: ticarcillin (53%), piperacillin (69%) (MIC 16 mg/L), ceftazidime (77%), cefepime (55%), cefpirome (40%), aztreonam (57.5%), imipenem (81.5%) (MIC 4 mg/L), amikacin (64.5%) (MIC 8 mg/L) and ciprofloxacin (58%) (MIC 1 mg/L). Resistance to beta-lactams was linked to the production of transferable beta-lactamases (30%), overproduction of cephalosporinase (29%) and to non-enzymic mechanisms (38%).

In vitro development of resistance of Streptococcus pneumoniae to beta-lactam antibiotics

In recent years, increasing numbers of Streptococcus pneumoniae strains displaying relative resistance to penicillin have been reported. Epidemiological studies have shown a correlation between aminopenicillin administration and resistance. We investigated the development of resistance in six strains (four sensitive and two intermediate-resistant to penicillin) by serial daily passages in subinhibitory concentrations of amoxicillin (AMX), amoxicillin + clavulanic acid (AMC), imipenem (IMP), cefixime (CFM), cefatrizine (CTZ), cefadroxil (CDX), and cefuroxime (CXM). MICs were determined by the macrodilution method in brain-heart broth for each daily passage. The number of daily passages needed to increase the MIC by a factor of 8 was achieved with AMX, AMC, and CFM for most of the strains after a mean of 24, 20, and 11 passages, respectively, and for one-third of the strains, with CDX, IMP, and CTZ after 11, 11, and 21 passages, respectively. Decreased susceptibility to breakpoints for intermediate-resistant S. pneumoniae populations was noted for all strains with CFM, AMX, and AMC after a mean of 10, 18, and 21 serial passages, respectively, and for four of five strains with IMP and CTZ after 12 and 13 passages. CTZ-, CDX-, and CXM-passaged variants had increased MIC values only for cephalosporins, while AMX-, AMC-, IMP-, and CFM-passaged variants exhibited increased MICs to all antibiotics tested. These in vitro data appear to be in agreement with epidemiological studies and warrant further exploration with respect to possible clinical implications.

[In vitro antibacterial activity of a new fluoroquinolone, sparfloxacin, against nosocomial bacteria, and concordance curve]

Minimal inhibitory concentrations (MICs) of sparfloxacin (SFX) were determined by agar dilution for 3,164 bacterial strains isolated in 10 university hospitals; in addition, antibiograms by agar diffusion were performed with 5 micrograms disks. Activity of SFX against nalidixic acid (NAL) susceptible (S) Enterobacteriaceae was close to that of other fluoroquinolones (FQ) (MIC 50 and 90: 0.06-0.5 microgram/ml); like for other FQ, this activity was reduced against NAL intermediate and resistant (R) Enterobacteriaceae (2-16). MICs of SFX against P. aeruginosa were between 0.12 and 16 (1-32). SFX had also a good activity against NAL-S A. baumannii (CMI < or = 0.25) but this activity is reduced against NAL-R Acinetobacter (16). SFX was highly active against Haemophilus (0.016-0.06) gonococci (0.008), meningococci (0.008) and B. catarrhalis (0.008-0.03). SFX showed activity superior to the currently available FQ against methicillin susceptible staphylococci (0.06); the resistant strains [8] are usually methicillin resistant. SFX is more effective against enterococci (0.5), streptococci (0.25-0.5) and particularly pneumococci (0.25-0.5) including penicillin-resistant strains. The coefficient correlation of the regression curve is 0.876; for MIC breakpoints of 1 and 2 micrograms/ml, zone diameter breakpoints should be 20 and 16 mm.

In-vitro development of resistance to beta-lactam antibiotics in Streptococcus pneumoniae

Selection of resistant mutants was induced in broth by exposing pneumococci to serial sub-inhibitory concentrations of various beta-lactam antibiotics. Aminopenicillins selected for resistance to themselves and to cephalosporins although cephalosporins tended to select for resistance to their own class, with the exception of cefixime which seems to select cross-resistant organisms.

Possible prevention of in vitro selection of resistant Streptococcus pneumoniae by beta-lactamase inhibitors

The development of resistance in vitro in five strains of Streptococcus pneumoniae (3 with full susceptibility and 2 with intermediate susceptibility to penicillin) was investigated by serial passages in the presence of subinhibitory concentrations of amoxicillin and ampicillin. At the end of passaging, MICs of antibiotics for all the strains increased by a factor of four or more, reaching at least intermediate levels. MICs of cephalosporins, ampicillin and amoxicillin increased for almost all variants obtained. Similar results were obtained with amoxicillin plus clavulanic acid at a ratio of 2:1 and at a constant concentration of 2 micrograms/ml, and with ampicillin plus sulbactam at a ratio 2:1. In contrast, no significant modification of MIC was seen with ampicillin plus sulbactam at a constant concentration of 4 micrograms/ml sulbactam. These results suggest interaction of sulbactam with penicillin binding proteins as described previously for other bacterial species, and merit further investigation.

[Current status of resistance of bacteria to pefloxacin in hospital units. Cross resistance with ofloxacin and ciprofloxacin. Results of a multicenter study]

The antibacterial activities of pefloxacin (PEF), ofloxacin (OFL), ciprofloxacin (CIP) were measured by agar diffusion (5 micrograms strength disks) against 6370 non duplicate clinical isolates collected in November 1991 in 40 hospitals from various areas in France. The MICs of PEF, OFL and CIP were determined by agar dilution against intermediate or resistant strains to one of the three antibiotics (inhibition zone < 16 mm for PEF, OFL, < 19 mm for CIP). In the Enterobacteriaceae the overall incidence of resistance to PEF (MIC > 4 mg/l) was 8% with important variations between the different species: E. coli 2%, Salmonella 0%, E. cloacae 9%, K. pneumoniae 21%, P. mirabilis 13%, P. vulgaris 3%, M. morganii 5%, Providencia 61%, S. marcescens 55%. Among S. aureus, the incidence of resistance was 3% for the methicillin-susceptible strains and 86% for the methicillin-resistant strains. The same applied to coagulase negative Staphylococci: methicillin-susceptible 9%, methicillin-resistant 62%. The frequency of the resistant strains was high among P. aeruginosa: 40% and A. baumanii: 83%. A high degree of correlation was observed between the MICs of PEF, OFL and CIP for all the bacterial species: r = 0.8 - 0.9. No major discrepancies were noted between the clinical categorizations of the activities of the three antibiotics for the different species except for P. aeruginosa: 4% of the strains were resistant to pefloxacin and susceptible to ciprofloxacin.

Comparison of phenotypic methods and DNA hybridization for detection of methicillin-resistant Staphylococcus aureus

One hundred thirty-eight Staphylococcus aureus isolates from patients with severe staphylococcal infections were collected in 15 French hospitals. Detection of the mec gene was performed by dot blot hybridization with a specific DNA probe. Dot blot results were used to characterize the isolates as methicillin susceptible (77 isolates) or resistant (61 isolates). The isolates were screened for methicillin resistance by an agar spread method on Mueller-Hinton plates containing oxacillin (2 and 10 micrograms/ml) and were incubated at 37 degrees C, with 10(8) CFU as the inoculum. MICs of oxacillin and methicillin were determined by the agar dilution method on Mueller-Hinton plates without NaCl, by using 10(5) CFU per spot, after 24 and 48h of incubation at 30 or 37 degrees C. Moderately elevated MICs were found for 20 isolates (14.5%). The mec gene was detected in six (30%) of the isolates expressing a low level of resistance to methicillin and/or oxacillin. As determined by comparison with probe hybridization results, the spread plate method with oxacillin at 2 micrograms/ml was more sensitive (sensitivity, 100%) and specific (specificity, 100%) than agar dilution with either methicillin or oxacillin in identifying methicillin resistance or susceptibility. Determinations of methicillin and oxacillin MICs by the agar dilution method had a specificity of 99 to 100% depending on the conditions of incubation, but the sensitivity was below 85% whatever the duration or temperature of incubation.

Conditions for the emergence of resistance to cefpirome and ceftazidime in experimental endocarditis due to Pseudomonas aeruginosa

The conditions for the emergence of resistance to cefpirome and ceftazidime were studied in rabbits with experimental aortic endocarditis due to Pseudomonas aeruginosa. The MIC of cefpirome was 16 mg/L and that of ceftazidime was 4 mg/L. Resistant mutants with MICs of > or = 64 mg/L were obtained in vitro to cefpirome after a single passage and to ceftazidime after five passages. A single dose of 50 mg/kg intramuscularly gave mean peak serum concentrations of 110.0 +/- 31.7 mg/L for cefpirome compared with 67.7 +/- 21.4 mg/L for ceftazidime and the half-lives were 1.2 +/- 0.1 h and 2.1 +/- 0.4 h, respectively. After treating infected rabbits for 4 days with various dosing regimens, resistant strains were only detected in those animals in which the time that the serum concentration exceeded the MIC was less than half of the dosing interval. There was no evidence of emergent resistance when the serum concentrations exceeded the MIC for a longer period nor when amikican was combined with the cephalosporins on the first day of therapy. Moreover, once differences in MICs and pharmacokinetics were taken into account, both antibiotics had a similar propensity to induce resistance.

Interactions of ceftibuten with extended-spectrum beta-lactamases: a bacteriological and enzymatic analysis

The authors analysed the antibacterial activity of ceftibuten, cefotaxime, ceftazidime and aztreonam against Klebsiella pneumoniae strains, including those which produced novel extended-spectrum beta-lactamases. These molecules were also tested for their susceptibility to cell-free extracts of the corresponding beta-lactamases. Both approaches showed that ceftibuten was not hydrolysed by the CTX-1/TEM-3, SHV-2 and SHV-3 beta-lactamases, while cefotaxime, ceftazidime and aztreonam were hydrolysed. Nevertheless all compounds were substrates for the SHV-4 and SHV-5 beta-lactamases, and the organisms which produced these beta-lactamases showed increased MICs.

In-vitro antibacterial activity of cefepime: a multicentre study

The antimicrobial activity of cefepime, a new broad-spectrum parenteral cephalosporin, was evaluated in vitro against 1757 recent clinical Gram-positive and Gram-negative isolates. Cefepime was active at low concentrations (MIC50 values < or = 0.06 mg/L and MIC90 values < or = 0.12 mg/L) against non-cephalosporinase-producing Enterobacteriaceae (Escherichia coli, Proteus mirabilis, Salmonella spp. and Shigella spp.). For Klebsiella pneumoniae, MICs were between 0.016 and 16 mg/L; the highest MIC values were observed for extended-spectrum beta-lactamase-producing strains. Against Enterobacteriaceae, such as cephalosporinase producing Enterobacter cloacae, MICs were < or = 0.5 mg/L, but MICs against cephalosporinase hyperproducing strains were generally higher. Ticarcillin-sensitive strains of Pseudomonas aeruginosa were inhibited by cefepime concentrations of 0.5-16 mg/L, while cefepime MICs were 8-64 mg/L for strains resistant to ticarcillin. The cefepime MIC50 value for Haemophilus spp. including many resistant to amoxycillin, was 0.03 mg/L. Against methicillin-sensitive strains of Staphylococcus aureus, cefepime MICs were 0.5-16 mg/L; MICs against methicillin-resistant staphylococci were 16- > 128 mg/L). Against methicillin-sensitive coagulase-negative staphylococci, cefepime MIC values were 0.03-16 mg/L; corresponding values for methicillin-resistant strains were 2-128 mg/L. Streptococci (Groups A, C and G) were sensitive to cefepime with MICs ranging from < or = 0.008-2 mg/L (MIC50, 0.03 mg/L; MIC90, 0.25 mg/L). The activity of cefepime against Group B streptococci and pneumococci were comparable, with MIC50 values of 0.12 and 0.25 mg/L, respectively, and MIC90 values of 0.03 and 0.25 mg/L, respectively. Most enterococci and all Listeria monocytogenes strains had MICs > or = 32 mg/L.(ABSTRACT TRUNCATED AT 250 WORDS)

A collaborative study of the in-vitro sensitivity to RP 59500 of bacteria isolated in seven hospitals in France

The in-vitro activity of RP 59500 was determined against 1051 recent clinical bacterial isolates. The susceptibility to RP 59500 was determined with an agar dilution technique for all the isolates, while MICs and MBCs were determined for 82 selected strains in broth. Isolates of both Staphylococcus aureus and coagulase-negative staphylococci appeared to be potentially susceptible to RP 59500, independent of susceptibility to methicillin or MLS resistance. (S. aureus: methicillin-sensitive, MIC90, 1.0 mg/L; methicillin-resistant, MIC90 1.0 mg/L; coagulase-negative staphylococci: methicillin-sensitive, MIC90 0.5 mg/L). Lancefield group A, B, C and G streptococci (MIC50 0.5 and MIC90 1.0 mg/L) and Streptococcus pneumoniae (MIC50 0.5 and MIC90 1.0 mg/L) appeared to be susceptible to RP 59500. Some Streptococcus spp. and enterococci as well as Listeria monocytogenes were inhibited by a higher concentration of RP 59500 (enterococci: MIC90 4 mg/L, range 0.125-16 mg/L). Comparatively low MICs were seen when Legionella spp., Neisseria gonorrhoeae and Gardnerella vaginalis were tested. Broth dilution MIC/MBC determinations showed no evidence of tolerance, as MIC values were within two dilutions of MBC values. RP 59500 might be a useful compound in the treatment of infections caused by a range of Gram-negative and Gram-positive bacteria, including those resistant to methicillin and/or macrolides.

[In vitro activity of tazobactam and piperacillin combination against 224 strains of Pseudomonas aeruginosa according to the production of beta-lactamase]

Piperacillin (PIP) alone and combined with 4 mg/l, 8 mg/l of tazobactam (TAZ) were tested by MIC determination on Mueller-Hinton agar against 224 clinical strains of P. aeruginosa: "wild type" (BLA-), 32 producing a constitutive cephalosporinase (CEP), 41 producing the PSE-1 type beta-lactamase, 7 PSE-2, 8 PSE-3, 9 PSE-4, 13 TEM-1, 24 TEM-2, 13 OXA-1, 22 OXA-2, 5 OXA-3. The combination with 8 mg/l was more effective than that one with 4 mg/l. Combinations of PIP-TAZ 8 mg/l reduced the MICs of PIP for the resistant strains (MICs greater than 64 mg/l) to the susceptible ot the moderately susceptible range (MICs less than or equal to 64 mg/l) for 31% of the CEP producing strains, 63% of the PSE-1, 15% of the PSE-2, none of the PSE-3, 34% of the PSE-4, 39% of the TEM-1, 30% of the TEM-2, 23% of the OXA-1, 14% of the OXA-2, 27% of the OXA-3, TAZ is the first beta-lactamase inhibitor effective against the constitutive cephalosporinase of P. aeruginosa; it is also very effective against the most frequently found PSE-1 beta-lactamase in P. aeruginosa.

[In vitro activity of beta-lactam antibiotics in combination with sulbactam against enterobacteriaceae, Pseudomonas aeruginosa and Acinetobacter. Results of a multicenter study]

The in vitro antimicrobial activity of ticarcillin (TICAR), mezlocillin (MEZLO), piperacillin (PIPER), cefoperazone (CPZ), cefotaxime (CTX) and ceftazidime (CAZ), alone and in combination with 8 micrograms/ml of sulbactam (SULB), was studied by agar dilution against TICAR resistant strains isolated in 8 hospitals over a period of 3 months in 1989 (747 enterobacteria, 110 Ps. aeruginosa and 48 Acinetobacter sp.). SULB did not modify the activity of beta-lactam antibiotics against Ps. aeruginosa. The 6 beta-lactam antibiotics SULB combinations were only active for 27% of Acinetobacter SULB sensitive. SULB restored the activity of: MEZLO, PIPER, CPZ in Enterobacteria producing a penicillinase; PIPER, CTX and CAZ in Enterobacteria producing a broad-spectrum beta-lactamase; MEZLO, PIPER, CTX and CAZ in M. morganii producing a derepressed cephalosporinase.

[Frequency and distribution of beta-lactamases in 1792 strains of Klebsiella pneumoniae in France between 1985 and 1988]

In october 1985, 1987 and 1988, all the clinical isolates of K. pneumoniae (respectively 530, 654, 590 strains) were collected in 20 hospitals. The beta-lactamases were identified by analytical isoelectrofocusing and by substrate and inhibition profiles. 76 to 81% of the strains produced only one beta-lactamase: SHV-1 type, pI 7.7 (61 to 65%) or PI 7.1 (14%). The TEM-1 betalactamase (pI 5.4) was produced in 1985 by 21% of the strains, 9% in 1987, and 11% in 1988: TEM-2, pI 5.6 by 2% in 1985-87-88. The extended broad spectrum beta-lactamases, able to hydrolyse amino-thiazol-oximino-beta-lactam antibiotics, TEM or SHV type enzymes (SHV-2, pI 7.7, SHV-3, pI 7.1; SHV-4/CAZ-5, pI 7.8; SHV-5/CAZ-4 pI 8.2; CTX-1/TEM-3, pI 6.3) were also detected: 0.75% of the strain (3 strains) in 1985, 8.4% (55 strains) in 1987, 11% (65 strains) in 1988. These extended broad spectrum beta-lactamases were found in 2 hospitals in 1985, 10 in 1987 and 9 in 1988.

[In vitro antibacterial activity of lomefloxacin, a new fluoroquinolone, against hospital strains. Results of a multicenter study]

Minimal inhibitory concentrations (MICs) of lomefloxacin (LOM) was determined by agar dilution for 2,819 bacterial strains isolated in 1988 in 9 university hospitals. Activity of LOM against nalidixic acid (NAL) susceptible (S) Enterobacteriaceae was close to that of pefloxacin (PEF) (mode MIC: 0.12-0.25 micrograms/ml); like for PEF, this activity was reduced against NAL resistant (R) Enterobacteriaceae (mode MIC: 4). MICs of LOM against P. aeruginosa were between 0.5 and 16 (mode: 2). LOM had also a good activity against NAL S A. baumannii (mode MIC: 0.5) but this activity is reduced against NAL R Acinetobacter (MICs : 4 to 128). LOM was highly active against Haemophilus (mode MIC: 0.06), Gonococci (mode MIC: 0.008), Meningococci (mode MIC: 0.03), Branhamella (mode MIC: 0.12-0.25). LOM showed activity similar to PEF against methicillin susceptible Staphylococci (mode MIC: 0.5-1); the resistant strains are usually methicillin resistant. Similar to the currently available quinolones, LOM is less effective against L. monocytogenes (mode MIC: 8), Enterococci (mode MIC: 4), Streptococci (mode MIC: 4) and Pneumococci (mode MIC: 8). Finally, for the anaerobic bacteria, LOM is more active against Clostridium perfringens (mode MIC: 1) than against Bacteroides fragilis (mode MIC: 32).

[Bactericidal activity of imipenem]

The bactericidal activity of imipenem against Pseudomonas aeruginosa, Enterobacteriaceae and Staphylococcus aureus was compared with that of other antibiotics, using the time-related kill rate curve method. The activity of imipenem was found to be less rapid and less intense than that of aminoglycosides, similar to that of fluoroquinolones and more rapid than that of beta-lactam antibiotics, notably ticarcillin and ceftazidime against P. aeruginosa and cefotaxime against Enterobacteriaceae. As with aminoglycosides, this bactericidal activity was concentration-dependent against Enterobacteriaceae and S. aureus but not against P. aeruginosa. Unlike the beta-lactam antibiotics, but like aminoglycosides and fluoroquinolones, imipenem remained active against "quiescent" P. aeruginosa, though not against Enterobacteriaceae. Successive transfers into liquid media rapidly produced resistant variants among P. aeruginosa strains and methicillin-resistant S. aureus strains, but not among Enterobacteriaceae. This shows that the bactericidal activity of imipenem is markedly different from that of other beta-lactam antibiotics.

[In vitro effects of combinations of teicoplanin and ceftriaxone against Staphylococcus aureus and Enterococcus]

The in vitro activity of antibiotic combinations of teicoplanin (T) and ceftriaxone (C) was evaluated on 10 methicillin sensitive S. aureus (MSSA), 10 methicillin resistant S. aureus (MRSA), 4 Enterococcus and 2 E. coli. By the checkerboard method the combination T + C showed synergism against 9 MRSA, 2 MSSA, 4 Enterococcus, indifferent effect against the other strains. By the time-kill-curves method synergism was found for the combinations T + C at 1/2 CMI against MRSA, MSSA and Enterococcus, and indifference with the combinations at 1 X CMI, 2 X CMI, 4 X CMI against all the strains. Antagonistic effect was never found. These results suggest that teicoplanin in combination with ceftriaxone can be tested in in vivo study to demonstrate the possible benefits of the synergism particularly in the therapy of the nosocomial infections.

[Comparative bactericidal activity of fourteen antibiotics against Staphylococcus aureus]

The bactericidal activity of LY 146032 (LY), Oxacillin (OXA), Cefamandole (CEF), Rifampin (RIF), Gentamicin (GEN) or Tobramycin (TOB), Pefloxacin (PEF), Vancomycin (VAN), Teicoplanin (TEI), Pristinamycin (PRI) was compared against 8 strains of S. aureus (4 Meth. sensitive, 4 Meth. resistant). Kill Kinetics studies were done: bacteria were incubated with antibiotics at their MICs, 2 X MIC, 4 X MIC and at concentrations obtained in vivo with usual therapeutic doses. With 4 X MIC, a 3 Log reduction of the initial inoculum was observed only at 24 h with OXA, CEF (MSSA), RIF, PEF, VAN, at 30 h with TEI and PRI. With LY, GEN or TOB at 2 X MIC the 3 Log reduction was observed at 3 h or 4 h, a greater than or equal to 5 Log reduction at 24 h: LY and Aminoglycosides are the most bactericidal antibiotics against S. aureus.

[In vitro antibacterial activity of a new macrolide, miokamycin. Results of a multicenter study]

Minimal inhibitory concentration (MIC) of miokamycin (M) were evaluated by agar dilution for 1,024 bacterial strains isolated in 6 hospitals and classed as a function of susceptibility and resistance to macrolides, lincosamides, streptogramins group (MLS). MIC of M ranged from 0.25 to 4 micrograms/ml (mode MIC 1-2) on Staphylococcus susceptible to MLS and on MLSB inducible strains; M was inactive on MLSB constitutive strains. MIC of M ranged from 0.016 to 4 micrograms/ml (mode MIC 0.12 to 0.5) for Streptococci and Pneumococci susceptible to erythromycin (E) and from 0.12 to greater than 128 for strains resistant to E. Enterococci susceptible to E were inhibited by 0.5 to 2 micrograms/ml (mode MIC 1) and strains resistant to E by 4 to greater than 128. Haemophilus were inhibited by 2 to 64 micrograms/ml (mode MIC 32), Neisseria by 0.12 to 4 (mode MIC 0.5-1) and B. catarrhalis by 0.12 to 8 (mode MIC 1). L. pneumophila was very susceptible to M: MIC 0.016 to 0.12 (mode MIC 0.06). MIC of M ranged generally from 0.5 to 2 micrograms/ml (mode MIC 1) for C. perfringens and from 0.03 to 2 (mode MIC 1) for B. fragilis. Thus, M was shown to be among macrolide antibiotics of resistance non-inducing type on MLSB inducible resistance strains. Its activity was similar to that of spiramycin slightly superior on Staphylococci, slightly inferior on Streptococci and Enterococci, similar on Pneumococci, very superior on Neisseria, Legionella and anaerobes. M had a good activity on Branhamella and, as others macrolides, was poorly active on Haemophilus.

[Sensitivity of Pseudomonas aeruginosa and Klebsiella spp. to ceftazidime. Current status in France]

Ceftazidime was tested against 2,224 strains of Pseudomonas aeruginosa obtained from 17 hospitals in April, May and June, 1986 and against 607 strains of Klebsiella pneumoniae and 234 strains of K. oxytoca obtained from 16 hospitals in October, 1987. The MIC's of ceftazidime against P. aeruginosa were distributed normally, with an MIC50 of 2 mg/l and an MIC90 of 4 mg/l. Depending on critical concentrations, 80 per cent of strains were sensitive, 11.4 per cent were of intermediate sensitivity and 0.54 per cent were resistant. There were few differences in results between hospitals. Ninety-two per cent of resistant strains and 45 per cent of intermediate strains (as opposed to 6 per cent of all strains) produced a high-level constitutive cephalosporinase with little variations between centres. The MIC's of ceftazidime against K. pneumoniae and K. oxytoca had a bimodal distribution: 91 per cent of strains were sensitive to 0.25 mg/l, 6 per cent of strains showed intermediate sensitivity and 3 per cent were resistant. All intermediate and resistant strains produced a very broad spectrum beta-lactamase which hydrolyzed some of the third generation cephalosporins: K. pneumoniae 36 CTX-1, 5 SHV-2, and 14 strains producing a recently identified beta-lactamase "CAZ-5/SHV-4"; K. oxytoca 3 CTX-1. These strains were isolated in 10 of the 16 hospitals which took part in the 1987 study. Comparison of these results with those of studies performed in 1984 and 1985 showed a moderate increase in the number of intermediate sensitivity strains of P. aeruginosa and the occasional occurrence, of the epidemic type, in some hospitals of Klebsiella spp. producing very broad spectrum beta-lactamases which were rare in 1985.

[In vitro bactericidal effect of cefotetan-aminoside combinations]

Bactericidal activity of cefotetan-gentamicin combinations was studied on 6 bacterial strains: S. aureus, K. pneumoniae, E. cloacae, S. marcescens, P. vulgaris, P. stuarti. Time kille curves technic was performed with final concentrations of cefotetan: 4 at 32 mg/l, and of gentamicin: 0.25; 0.5; 1; 2; 8 mg/l and with an 10(6) CFU/ml inoculum. Cefotetan at 4 mg/l was not able to obtain a 0.01% (percentage of survivors) bactericidal activity before 24 h. The combination cefotetan-gentamicin (0.25 to 2 mg/l according to the strains) were bactericidal (0.01% of survivors) before 24 h: 1 to 6 h according to the strains, more rapidly than with gentamicin alone at the same concentration. This more rapid bactericidal activity obtained by cefotetan-gentamicin combination seems to indicate this combination in the treatment of severe infections in immunocompromised patients.

[Comparative bactericidal activity of imipenem and other antibiotics against Pseudomonas aeruginosa]

The bactericidal activity of imipenem (IM), ticarcillin (TIC), ceftazidime (CEZ), amikacin (AMK) or tobramycin (TOB), ciprofloxacin (CIP), was compared on 6 P. aeruginosa: 1 ticarcillin, susceptible strain (TICs), 5 constitutive beta-lactamases producing strains (PSE, TEM, OXA1, OXA2, cephalosporinase (CEP). The time-kill-curves method was performed. Bacteria were incubated with antibiotics at M.I.C. X2 and at concentrations obtained in vivo with usual therapeutic doses: IM 4-8 mg/l, TIC 64-128, CEZ 4-32, TOB 2-8, AMK 4-16, CIP 1-4. The bactericidal activity of IM was independent of the concentration. A 5 Log10 reduction in viability was observed at 3 h for strains TICs, TEM and PSE, 3 Log10 for strains OXA and CEP. This bactericidal activity was also observed in a non growing system. The bactericidal activity of TIC (strain TICs) and CEZ was slower and weaker: 1-2 Log10 at 6 h, 2-3 Log10 at 24 h. It is not observed in a non growing system. On the other hand, the bactericidal activity of aminoglycosides and ciprofloxacin was rapid: 3-4 Log10 at 3 h, 5 Log10 at 6 h for all strains in growing and non growing systems. IM is the only beta-lactam antibiotic bactericidal on growing and non growing cells of P. aeruginosa like aminoglycosides and new quinolones.

[Methods for in vitro studies of antibiotic combinations. Indications and limits]

Several methods are available in the bacteriology laboratory to study the antibacterial effect of antibiotic combinations. With "end point" methods, results are read and interpreted after 18 or 24 hours of incubation. Amongst these methods, gel diffusion is technically easy but gives only qualitative results. Respective concentrations in the interaction zone cannot be evaluated and are unrelated to concentrations obtained in vivo. In liquid medium, the "checker board" method offers the possibility of combining a large number of concentrations. Interpretation may be graphic or mathematical. It is often arbitrary. But above all these methods fail to recognize events which occur during the 18 or 24 hours of incubation. Kinetic methods can be used to evaluate the kinetics of bactericidal action in relation to time and hence to compare the rapidity and degree of the bactericidal action of antibiotics used alone or in combination. However, only a limited number of concentrations can be studied. During the evaluation of a new antibiotic all methods must be tried to determine the possible synergistic or antagonistic effects of combinations regarding bacteria with known different resistance mechanisms. The broad outline thus obtained serves as a prescribing base for combinations. Bactericidal effect should be confirmed by a kinetic method where the concentrations in contact with the bacterial inoculum are selected in relation to presumed concentrations obtained at the site of infection.

[Beta-lactam combinations with new quinolones]

The combination of new quinolones with beta-lactam antibiotics can be an alternative to the classical use of aminoglycoside-beta-lactam antibiotic combinations. In vitro studies, in particular using cefotaxime-pefloxacin or ofloxacin combinations against Enterobacteria and cefsulodin or ceftazidime with pefloxacin or ofloxacin against P. aeruginosa rarely show evidence of synergistic antibacterial activity when the "static" checker board method is used. In contrast, kinetic bactericidal studies very often show a notable increase in the rapidity of bactericidal activity as well as the disappearance of the frequent secondary regrowth seen after 6 hours with the antibiotics used alone. Techniques studying the development of resistance in vitro show that the combination of pefloxacin-cefotaxime against enterobacteria and pefloxacin-cefsulodin against P. aeruginosa are capable of inhibiting the emergence of resistant variants. These promising results require confirmation by studies using animal models and, above all, by the clinico-microbiological analysis of the results of clinical trials.

[Characteristics of penicillinase-producing strains of Neisseria gonorrhoeae isolated in France, 1979-1986]

Penicillinase producing Neisseria gonorrhoeae PPNG, had been first isolated in France in 1979. Since, they regularly increased if we considered France on the whole. From 1979 to 1986, 284 strains had been collected by a multicentric group. The frequency of isolation was strongly different in France, unknown in some region they rose 12% in specific areas in Paris. The PPNG strains were more frequently isolated from male than female (sex ratio was higher with PPNG than for non producing strain). They were more often responsible of complicated infections in female than male at the same rate than the non producing strains. Auxotype distribution was different between producing and non producing strains. Plasmidic content from african type (Af) was almost the same than from asian type (As). Strains with Af type associated with the conjugative plasmid were increasing.

[Aminoside sensitivity of bacteria isolated in 1984 at the military hospital complex in the Paris region]

The minimal inhibiting concentrations (MIC) of 5 aminosids have been determined by the microdilution method in liquid medium of 4,582 bacterial strains isolated from various pathological samples: 1,039 Staphylococcus, 2,629 Enterobacteria, 759 Pseudomonas and 155 Acinetobacter. The phenotype of resistance has been defined for each strain by listing the antibiotics for which a resistance was observed. The frequencies of the bacterial resistances varied according to the aminosid (gentamicin, sisomicin, tobramycin, netilmicin and amikacin) and to the species studied. For the bacterial species studied, these frequencies of resistance to the aminosid family of antibiotic were weaker in the military hospitals in the district of Paris, than in different civil hospitals where similar studies were conducted.

[Antibiotic therapy of Pseudomonas aeruginosa infections]

Antibiotics active against Pseudomonas aeruginosa are carboxy- and ureidopenicillins, some cephalosporins, aztreonam, imipenem, aminoglycosides, fluoroquinolones and, in some special circumstances, polymyxins and fosfomycin. The choice of the antibiotic should be based not only on minimal inhibitory concentrations in vitro, but also on the inoculum effect, on the speed with which bacteria are killed, on natural or acquired resistance and on data obtained from experimental infections in animals. Antibiotics used against P. aeruginosa infections must be rapidly bactericidal. Since the prognosis is severe, the initial treatment given before the sensitivity of the strain is known must combine a beta-lactam antibiotic and an aminoglycoside, both being selected among the most regularly active.

[Present status of the sensitivity of Pseudomonas aeruginosa to aminoglycosides in France]

9,119 strains of P. aeruginosa were collected from 10 French hospitals from 1981 to 1984. The MIC of gentamicin (GM), sisomicin (SIS), tobramycin (TOB), dibekacin (DBK), netilmicin (NET), amikacin (AMK), and habekacin (HBK) was determined by the two fold microdilution method. 81% of the strains were susceptible to the 4 aminoglycosides. The MICs (geometric mean, mg/l) for these susceptible strains were: GM: 1.69, SIS: 1.58, TOB: 1.06, DBK: 1.12, NET: 2.73, AMK: 4.16, HBK: 2.70. 19% of the 9 119 strains were resistant to SIS, 18% to GM, 16% to TOB and DBK, 11% to NET, 3% to AMK and HBK. Among the 1 758 strains resistant to one or several aminoglycosides, the frequency of the resistance phenotypes was GM-SIS: 11%, GM-SIS-NET: 4.95%, GM-SIS-TOB-DBK: 32.7%, GM-SIS-TOB-DBK-NET: 34.45%, SIS-TOB-DBK-NET: 2.5%, SIS-TOB-DBK-NET-AMK-HBK: 2.39%, GM-SIS-TOB-DBK-NET-AMK-HBK: 11.61%. The mechanism of resistance of the phenotypes 1 to 6 is enzymatic. The mechanism of resistance of the phenotype 7 (resistance to the six aminoglycosides) is either enzymatic or due to decreased uptake or penetration of antibiotic. HBK and AMK are the aminoglycosides most frequently active against P. aeruginosa. The patterns of resistance to both antibiotics are the same. On a weight-for-weight basis, HBK is more active than AMK.

[In-vivo bacteriostatic and bactericidal activity of pefloxacine]

After oral ingestion of pefloxacin (400 mg every 12 hours for four days), the following characteristics were studied: bacteriostatic and bactericidal activity of urine for controls free of urinary tract infection on 6 "test" strains; kinetics of bacteriostatic and bactericidal activity of urine as compared to serum in patients with urinary tract infection. The urinary bacteriostatic level approximates bactericidal levels in healthy individuals. This level, that varies across bacterial species, remains steady between the third hour and the twenty-fourth hour following initiation of treatment: values found were 1/16 to 1/8192 for enterobacteria, 1/64 to 1/256 for Pseudomonas aeruginosa and 1/256 to 1/512 for Staphylococcus aureus. In patients, urinary killing curves show that bacteriuria decreases by 5 Log10 approximately, starting at the third hour. Urine was sterile by the sixth hour. In most cases, serum bacteriostatic activity was above 1/32 throughout treatment, with maximum levels being recorded on the fourth day (above 1/64). Bactericidal activity is similar to bacteriostatic activity.

[Multicenter study of the activity of pefloxacin on bacteria isolated in a hospital milieu]

Minimal inhibitory concentrations (MICs) of pefloxacin were evaluated by agar dilution for 3422 bacterial strains isolated in nine hospitals. Enterobacteriaceae proved very sensitive to pefloxacin: 62 p. 100 of 1743 strains tested had MIC less than or equal to 0.12 and 90 p. 100 less than or equal to 1 microgram/ml; but the percent of strains with MIC greater than or equal to 2 varied among the different groups of Enterobacteriaceae: 2.7 p. 100 for E. coli to 39 p. 100 for Serratia. 55 p. 100 for Pseudomonas and 81 p. 100 of Acinetobacter were inhibited by 1 micrograms/ml or less (mode MIC 1 and 0.5 micrograms/ml). Haemophilus sp.: 0.03 and 0.06 micrograms/ml and Gonococci were very sensitive to pefloxacin. The spectrum of pefloxacin extended to Gram positive cocci: MIC of Staphylococci were 0.06 to 8 micrograms/ml (mode MIC: 0.5); Enterococci, other Streptococci and Pneumococci were less sensitive: 2 and 4 micrograms/ml for the majority of strains. Concerning anaerobic bacteria, pefloxacin was more active against Clostridium (0.5 to 1 microgram/ml generally), than against Bacteroides (4 to 16 micrograms/ml).

In-vitro comparison of macrolides, lincosamides and synergistins on Neisseria gonorrhoeae

The MIC of erythromycin, oleandomycin, spiramycin, josamycin, lincomycin and pristinamycin was determined for 100 strains of Neisseria gonorrhoeae isolated from cases of acute urethritis in men. The method of dilution in agar was used: blood agar with the addition of 'Polyvitex' and an innoculum of 10(3)-10(4) bacteria per spot. With respect to the break points defined by the C.F.A. all the strains of N. gonorrhoeae studied are sensitive to erythromycin, spiramycin, josamycin and pristinamycin, 12% strains are resistant to oleandomycin and 75% to lincomycin. The active antibiotics are classified as follows according to the active weight expressed by the MIC50: erythromycin, pristinamycin, 0.125 mg/l; josamycin, 0.5 mg/l; spiramycin, oleandomycin, 2 mg/l.

[In vitro activity of ceftriaxone on hospital bacteria. Results of a multicenter study]

Minimal inhibitory concentrations (MICs) of ceftriaxone were determined by agar dilution for 2 099 strains isolated in six teaching hospitals. MICs were less than 1 microgram/ml for the great majority of Enterobacteriaceae, with mode MICs varying across groups from less than 0.008 micrograms/ml for Proteus (mirabilis and indole-positive) to 0.25 for Enterobacter. Only a few resistant strains were found, mainly among Enterobacter and Citrobacter. Ceftriaxone proved noticeably less active against P. aeruginosa and Acinetobacter (mode MICs: 16 micrograms/ml). Haemophilus sp. and Gonococci, regardless of beta-lactamase production status, as well as Neisseria meningitidis, were highly susceptible (MIC less than 0.008-0.032). Ceftriaxone was moderately active against methicillin-susceptible staphylococci (MIC: 2 to 8 micrograms/ml) and failed to inhibit methicillin resistant strains. Enterococci were slightly susceptible or resistant, whereas the other Streptococci and Pneumococci had low MICs (0.03-0.25). A fairly wide range of MICs was found for anaerobes (Clostridium: 0.06-2, Bacteroides: 0.5-32). Our data show that its particularly strong activity against Proteus, Haemophilus and Neisseria sets ceftriaxone apart from the other third-generation cephalosporins.

[Multicenter study of the in vitro effect of imipenem (N-formimidoyl-thienamycin) on hospital bacteria]

Minimal inhibitory concentrations (MICs) of imipenem were evaluated by agar dilution for 2 895 bacterial strains isolated in 9 hospitals. Imipenem proved highly active against Enterobacteriaceae, with an MIC less than or equal to 0.25 for 63% of the 1 556 tested strains, less than or equal to 1 for 89.6% and less than or equal to 4 for 99%. The different groups of Enterobacteriaceae exhibited similar mode MICs (0.12 to 0.25), with the exception of Serratia (0.25-0.5), P. mirabilis (0.5), indole-positive Proteus (2), and Providencia (1). MICs of most cefotaxime-resistant strains were within the susceptibility range. Imipenem also exhibited satisfactory activity against P. aeruginosa (mode MIC 1-2) and Acinetobacter sp. (mode MIC: 0.25-0.5). MICs ranged from 0.03 to 4 (mode MIC: 0.5) for Haemophilus sp. and 0.25 to 1 for Gonococci, regardless of beta-lactamase-production status. MICs for Meningococci were less than or equal to 0,06. Methicillin-susceptible Staphylococci had low MICs, ranging from 0.008 to 0.5 (mode MIC : 0.016); MICs for methicillin-resistant strains varied widely, from 0.016 to 64, and were higher after incubation at 30 degrees C. Streptococci, except for Enterococci, and Pneumococci were highly susceptible (usually 0.008-0.03); MICs for Enterococci varied from 0,12 to 32 (mode MIC: 1-2). Except for four C. difficile strains, all tested anaerobic strains were inhibited by concentrations less than or equal to 1 (mode MICs: 0.06 for C. perfringens and 0.03 for B. fragilis).

[In vitro study of the effects of a ticarcillin-clavulanic acid combination on Pseudomonas aeruginosa as a function of resistant phenotypes]

Activity of ticarcillin combined with clavulanic acid against 203 P. aeruginosa strains was studied. 159 strains produced a constitutive beta-lactamase with resistance to ticarcillin (MIC less than 128 mg/l) as a result. Minimal inhibitory concentrations (MICs) were determined using the agar dilution method for ticarcillin alone and combined with 4 and 8 mg/l clavulanic acid. Addition of clavulanic acid failed to change the activity of ticarcillin on ticarcillin-susceptible stains and on strains producing a constitutive cephalosporinase. A synergic effect was demonstrated for PSE and TEM type beta-lactamase producers and for OXA type beta-lactamase producers (4 halving dilutions and 1 to 2 halving dilutions respectively). However, because of the levels of baseline MICs, the combination brought the MIC of ticarcillin below the cutoff level for a small percentage of PSE strains (6%), moderate percentage of OXA strains (17 to 50%) and significant percentage of TEM strains. These percentages were slightly higher with 8 mg than 4 mg clavulanic acid. Given current data on the prevalence of the various beta-lactamases among resistant strains now being isolated in France, the ticarcillin + clavulanic acid combination will enable to achieve an MIC less than 128 mg/l for 11% (with 4 mg) or 16% (with 8 mg) of resistant strains.

[Comparative in vitro activity of 2 monobactams (RO 172301 (AMA 1080)and aztreonam), ceftazidime and cefotaxime on Gram-negative bacilli]

In vitro activities of two monobactams (RO 172301 and aztreonam), ceftazidime and cefotaxime against 739 bacterial strains belonging to different species of Gram negative aero-anaerobic bacilli were studied comparatively. Strains were studied according to their resistance phenotype to beta-lactams. Minimal inhibitory concentrations (MIC) of the four antibiotics were determined using the agar dilution method. RO 172301 and aztreonam exhibited a similar activity against P. aeruginosa with a MIC around 2 mg/l for 50% of strains and around 4 mg/l for 90% of strains. As compared to the two monobactams, ceftazidime was more active and cefotaxime less active (one halving dilution either way). Activity of antibiotics was not dependent upon the mechanism of resistance to beta-lactams, except for strains producing a constitutive cephalosporinase. Against Enterobacteriaceae, RO 172301 proved slightly more active than aztreonam and cefotaxime; ceftazidime was the least active drug. Only 12 of the 739 strains tested (1.6%) had a MIC for RO 172301 above 32 mg/l. The percentage of resistant strains was 2.4% for aztreonam and ceftazidime and 8.8% for cefotaxime.

[Multicenter study of the antibacterial activity of a new cephalosporin: CM 40874]

Minimal inhibitory concentrations (MIC) of CM were evaluated on 2 548 bacterial strains isolated in 8 hospitals. CM demonstrated high activity on Enterobacteriaceae, the MIC being less than or equal to 0.125 micrograms/ml for 71% of the 1 362 strains tested, less than or equal to 1 for 99.6%, and less than or equal to 4 for 99.9%. Mode MIC varies little among the different groups of Enterobacteriaceae (from 0.06 to 0.12 micrograms/ml), with the exception of Serratia sp. (mode MIC : 0.25) and Klebsiella oxytoca (mode MIC : 0.03). Most of Enterobacter, Serratia, and Citrobacter sp. strains not inhibited by cefotaxime are readily inhibited by CM at the same concentrations than susceptible strains. CM has less activity on P. aeruginosa (MIC 2-32 micrograms/ml) and Acinetobacter sp. (MIC 8-128). Staphylococci (MIC 32) and Enterococci are not susceptible. Variable activity is found against other Streptococci. CM inhibits Haemophilus sp. at MICs of 0.12 to 0.5 micrograms/ml and Gonococci at MICs of 0.03 to 0.5 (whether the strains produce beta-lactamase or not). Meningococci have a mode MIC of 0.03 micrograms/ml (range 0.008 to 0.25). Thus, CM 40874 is a new third generation cephalosporin with high activity on Enterobacteriaceae, including those strains not susceptible to cefotaxime and good activity on Haemophilus sp. and Neisseria sp. This additional activity is probably supported by enhanced resistance to enzymatic inactivation by beta-lactamases.