Antimicrobial resistance among respiratory pathogens collected in Thailand during 1999-2000

A multi-center surveillance study was conducted in Thailand during 1999-2000 to determine antimicrobial susceptibilities among the respiratory pathogens Streptococcus pneumoniae (n = 206), Haemophilus influenzae (n = 305), and Moraxella catarrhalis (n = 39). Of the S. pneumoniae isolates collected, 33.5% were penicillin-susceptible, 27.2% intermediate and 39.3% resistant. Expectedly, resistance rates to beta-lactams were higher among penicillin-resistant (ceftriaxone, 14.8%; amoxicillin-clavulanate, 42.0%; cefuroxime, 100%) than penicillin-susceptible (ceftriaxone, 0%; amoxicillin-clavulanate, 0%; cefuroxime, 0%) isolates. Likewise, azithromycin and clarithromycin resistances were 4.3% and 5.8% among penicillin-susceptible isolates, and 77.8% and 95.1% among penicillin-resistant isolates. All S. pneumoniae remained susceptible to vancomycin and 99.5% were susceptible to levofloxacin. Multidrug resistance (resistance to >3 antimicrobial classes) was present in 25.2% of pneumococcal isolates (n = 52), with resistance to azithromycin, penicillin and trimethoprim-sulfamethoxazole the most common phenotype (40/52 isolates; 77.0%). Among the isolates of H. influenzae, the prevalence of beta-lactamase production was 45.2%. All isolates of H. influenzae were susceptible to amoxicillin-clavulanate, azithromycin, ceftriaxone, cefuroxime and levofloxacin while 49.5% were resistant to trimethoprim-sulfamethoxazole. All 39 isolates of M. catarrhalis produced beta-lactamase. Azithromycin (MIC90, < or = 0.03 microg/ml) and levofloxacin (MIC90, 0.03 microg/ml) were the most active agents tested against M. catarrhalis. The results of this study may serve as a baseline for future studies to monitor antimicrobial susceptibilities among respiratory pathogens in Thailand.     

Assessment of the susceptibility of Streptococcus pneumoniae to cefaclor and loracarbef in 13 countries

Between July 1998 and July 1999, 2,644 clinical isolates of Streptococcus pneumoniae were collected from 27 study centers in 13 countries and their susceptibilities to penicillin, cefaclor and loracarbef were determined by E-test" (AB BIODISK, Solna, Sweden). Overall, 96.3% of isolates were penicillin-susceptible (79.8%) or -intermediate (16.6%) (MIC, < or = 1 microg/ml). Rates of penicillin-resistant S. pneumoniae isolation varied widely and were highest in the study centers tested in New Zealand (10.9%), Canada (10.0%), Mexico (9.1%) and the United States (5.1%). Low rates of penicillin-resistance were found in the study centers tested in Russia (0%), Turkey (0%), Brazil (0.5%), Germany (0.6%), Philippines (1.6%), Italy (2.1%), United Kingdom (2.3%), Australia (3.0%) and Poland (3.1%). Using recently published NCCLS interpretative breakpoints (M100-S10, 2000), 87.2% (median) of all isolates tested were cefaclor-susceptible and 87.8% (median) of all isolates tested were loracarbef-susceptible. Of the penicillin-susceptible S. pneumoniae isolates, 99.5% were susceptible to both cefaclor and loracarbef. Susceptibility to cefaclor and loracarbef was also retained by 30.8% and 32.9% of penicillin-intermediate isolates, respectively. These findings are in contrast to recent publications reporting lower cefaclor and loracarbef activities using non-validated interpretative criteria. In conclusion, rates of penicillin resistance among recent clinical isolates of pneumococci remain low in many centers worldwide. Cefaclor and loracarbef demonstrated excellent in vitro activity against recent clinical isolates of penicillin-susceptible and many isolates of penicillin-intermediate S. pneumoniae.     

In-vitro activity of cefpodoxime proxetil (RU 51807): a comparative disk diffusion study on isolates from geriatric patients

997 strains isolated from clinical specimens arrived at the "Pio Albergo Trivulzio" microbiology laboratory were tested using disks of cefpodoxime, amoxicillin + clavulanic acid, cefaclor, cefuroxime, ofloxacin, cotrimoxazole, ceftriaxone and cefalexin. Gram-positive strains were tested also with erythromycin, while gram-negative bacteria were tested against aztreonam. Cefpodoxime overall activity was well above the effectiveness of the other oral cephalosporins and on the same order as ceftriaxone and ofloxacin. Cefpodoxime proved to be also more active than the combination amoxicillin-clavulanic acid.     

Assessment of the Susceptibility of Streptococcus pneumoniae to Cefaclor and Loracarbef in 13 Countries

Abstract

Between July 1998 and July 1999, 2,644 clinical isolates of Streptococcus pneumoniae were collected from 27 study centers in 13 countries and their susceptibilities to penicillin, cefaclor and loracarbef were determined by E-test” (AB BIODISK, Solna, Sweden). Overall, 96.3% of isolates were penicillin-susceptible (79.8%) or -intermediate (16.6%) (MIC, <1 μg/ml). Rates of penicillin-resistant S. pneumoniae isolation varied widely and were highest in the study centers tested in New Zealand (10.9%), Canada (10.0%), Mexico (9.1%) and the United States (5.1%). Low rates of penicillin-resistance were found in the study centers tested in Russia (0%), Turkey (0%), Brazil (0.5%), Germany (0.6%), Philippines (1.6%), Italy (2.1%), United Kingdom (2.3%), Australia (3.0%) and Poland (3.1%). Using recently published NCCLS interpretative breakpoints (M100-S10, 2000), 87.2% (median) of all isolates tested were cefaclor-susceptible and 87.8% (median) of all isolates tested were loracarbef-susceptible. Of the penicillin-susceptible S. pneumoniae isolates, 99.5% were susceptible to both cefaclor and loracarbef. Susceptibility to cefaclor and loracarbef was also retained by 30.8% and 32.9% of penicillin-intermediate isolates, respectively. These findings are in contrast to recent publications reporting lower cefaclor and loracarbef activities using non-validated interpretative criteria. In conclusion, rates of penicillin resistance among recent clinical isolates of pneumococci remain low in many centers worldwide. Cefaclor and loracarbef demonstrated excellent in vitro activity against recent clinical isolates of penicillin-susceptible and many isolates of penicillin-intermediate S. pneumoniae.     

Does the degree of penicillin susceptibility of Streptococcus pneumoniae affect the bactericidal activity of co-amoxiclav versus oral cephalosporins at physiological concentrations? An in vitro pharmacodynamic simulation

An in vitro model simulating amoxicillin-clavulanic acid (co-amoxiclav) versus oral cephalosporin serum concentrations was used to explore activity over time against penicillin-susceptible and non-susceptible Streptococcus pneumoniae. Initial inoculum reduction > 4 log cfu/ml (>99.9%) was obtained with co-amoxiclav against both strains. Cefuroxime, cefpodoxime and cefaclor achieved a similar reduction against the susceptible strain, but no reduction against the non-susceptible strain.     

Antimicrobial resistance among clinical isolates of Streptococcus pneumoniae isolated in four southern European countries (ARISE project) from adult patients: results from the cefditoren surveillance program

From four southern European countries (Spain, Italy, Portugal, and Greece) 877 Streptococcus pneumoniae isolates were recovered from adult patients with respiratory tract infections between September 2000 and March 2001. The antimicrobial susceptibility to 11 antibiotics was determined in a central laboratory. Penicillin resistance was high in Greece (47.1%) and Spain (25.1%) but much lower in Portugal (7.9%) and Italy (4.8%). On the other hand, erythromycin resistance was high in Italy (38.5%) and Spain (36.2%) with no statistical difference with Greece (29.4%) but reaching significance (p <0.01) with Portugal (15.7%). Resistance to levofloxacin was low (1.5%) but present in Spanish and Italian isolates. Cefditoren, a new cephem antibiotic tested, was the most potent compound (MIC90 = 0.5 microg/ml) followed by levofloxacin and cefotaxime (MIC90 = 1 microg/ml). Given the high rates of penicillin and macrolide resistance reported, there is an evident need for new drugs and continued antimicrobial surveillance of S. pneumoniae.     

Do Different Susceptibility Breakpoints Affect the Selection of Antimicrobials for Treatment of Uncomplicated Cystitis?

Abstract

An In Vitro model simulating amoxicillin-clavulanic acid (co-amoxiclav) versus oral cephalosporin serum concentrations was used to explore activity over time against penicillin-susceptible and non-susceptible Streptococcus pneumoniae. Initial inoculum reduction > 4 log cfu/ml (>99.9%) was obtained with coamoxiclav against both strains. Cefuroxime, cefpodoxime and cefaclor achieved a similar reduction against the susceptible strain, but no reduction against the non-susceptible strain.     

Antibiotic Susceptibility of Respiratory Pathogens Recently Isolated in Italy: Focus on Cefditoren

Abstract

The aim of this study was to evaluate the in vitro antibiotic susceptibility of respiratory pathogens recently isolated in italy to commonly used antibiotics including cefditoren. Six clinical microbiological laboratories collected, between January and September 2009, a total of 2,510 respiratory pathogens from subjects with community- acquired respiratory tract infections (CARTI). Cefditoren, out of all the beta-lactams studied, had the lowest MIC₉₀ against 965 strains of Streptococcus pneumoniae examined, followed by cefotaxime and ceftriaxone (2% resistance in penicillin-resistant S. pneumoniae (PRSP)). Against 470 Haemophilus influenzae, independently of their production of beta-lactamases or ampicillin resistance, cefditoren was the oral cephalosporin with the best in vitro activity, comparable to that of the injectable cephalosporins and levofloxacin. Higher MIC₉₀s were found for the macrolides (4 - 16 mg/L) and cefaclor (4 - 32 mg/L). As was foreseeable, Streptococcus pyogenes (225 strains) was uniformly sensitive to all the beta-lactam antibiotics, but the elevated miC90 values reduced (<75%) susceptibility of this pathogen to macrolides. Beta-lactamase-negative Moraxella catarrhalis (100 strains) had reduced susceptibility only to the macrolides, while the 250 beta-lactamase-producing strains also had reduced susceptibility to cefuroxime. Levofloxacin showed the lowest MIC₅₀/MIC₉₀ values in the producing strains, whereas cefditoren, cefotaxime and ceftriaxone in the non-producers. As regards the enterobacteriaceae, cefditoren and levofloxacin had the lowest MIC₉₀s against Klebsiella pneumoniae. Cefditoren and the third-generation injectable cephalosporins had the lowest MIC₉₀s against Escherichia coli (100% susceptibility) while levofloxacin was less active (86% susceptibility).

In conclusion, cefditoren's wide spectrum and high intrinsic activity, as well as its capacity to overcome most of the resistance that has become consolidated in some classes of antibiotics widely used as empiric therapy for CARTI, allows us to suggest that cefditoren might be included in the european guidelines as one of the first-choice antibiotics in the treatment of CARTI.     

Interrelationship between the pharmacokinetics and pharmacodynamics of cefaclor advanced formulation in patients with acute exacerbation of chronic bronchitis

Cefaclor advanced formulation (cefaclor AF) is an extended-release form of the oral cephalosporin cefaclor. When cefaclor AF 750 mg twice-daily and cefaclor immediate release 500 mg three-times-a-day are compared there is a skew to the right of the pharmacokinetic profile and higher levels are achieved. Based on this pharmacokinetic finding, we examined the relationship between the bacterial susceptibility to cefaclor (MIC), the achieved cefaclor AF serum and sputum concentrations, and in vivo eradication of the bacteria in 36 patients with acute exacerbations of chronic bronchitis. The mean peak concentrations in serum and sputum 5 h after administration were 8.6 microg/ml (95% CI: 8.1 microg/ml - 9.1 microg/ml) and 1.5 microg/ml (95% CI: 1.4 microg/ml - 1.7 microg/ml), respectively. Cefaclor was always detectable 8 h after administration. At post therapy, treatment was successful in 31 (86.1%) patients. Cefaclor concentrations in serum persisted above the MIC for more than 40% of dosing interval in 31 subjects, and those in sputum in 24 patients. Treatment was successful in all subjects with percent of time above the MIC in serum of >40%, whereas the time that levels in sputum stayed above the MIC was not the pharmacodynamic parameter that correlated best with therapeutic efficacy for cefaclor. Our data demonstrate that when cefaclor AF is dosed twice-daily, the in vivo pharmacodynamic susceptibility breakpoint is 8 microg/ml. The good activity and pharmacokinetics of cefaclor AF provide serum concentrations higher than the MIC of Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis for more than 40% of the validated dosing interval. Therefore, it might be considered for first choice treatment of acute exacerbations of chronic bronchitis.     

A pharmacodynamic model to support a 12-hour dosing interval for amoxicillin/sulbactam, a novel oral combination, in the treatment of community-acquired lower respiratory tract infections

We evaluated, by time-kill studies, the pharmacodynamics of amoxicillin/sulbactam (AMX/SUL, 875 mg/125 mg), a novel oral combination, against the major respiratory pathogens in 12 volunteers receiving a single dose. The sera corresponding to 50% of a 12-h dosing interval displayed either bactericidal or inhibitory activity against both a penicillin-susceptible and a penicillin-intermediate Streptococcus pneumoniae strain (penicillin MIC of 0.03 and 0.25 microg/ml, respectively), as well as against a beta-lactamase-positive Moraxella catarrhalis and a beta-lactamase-negative Haemophilus influenzae strain. Both the peak samples and those corresponding to 4 h after dose (i.e. 33% of a 12-h dosing interval) proved active against both a penicillin-resistant S. pneumoniae (MIC, 2 microg/ml) and a beta-lactamase-positive H. influenzae strain. The AMX-SUL formulation evaluated in this study showed pharmacodynamic features that support clinical trials to assess its efficacy in the treatment of lower respiratory tract infections with a 12-h dosing interval regimen.     

Update of the activity of cefditoren and comparator oral beta-lactam agents tested against community-acquired Streptococcus pneumoniae isolates (USA, 2004-2006)

Cefditoren and other orally administered cephalosporins are infrequently included in resistance surveillance studies. Here we evaluated 359 contemporary (2004-2006) strains of Streptococcus pneumoniae, including penicillin-intermediate (12.0%) and -resistant (22.8%) subsets from United States patients by reference broth microdilution methods. Cefditoren was the most potent cephalosporin tested (MIC(50), 0.015 mg/L), including against penicillin-intermediate strains (MIC(50), 0.12 mg/L), and was two-, four- and eight-fold more active than cefuroxime, cefdinir and cefprozil, respectively. Penicillin-resistant strains were largely resistant to all tested ss-lactams. We confirm the continued spectrum and potency for cefditoren against S. pneumoniae that surpasses that of other orally administered cephalosporins.     

Antibacterial Activity of Cefditoren Against Major Community-Acquired Respiratory Pathogens Recently Isolated in Italy

Abstract

In this study we evaluated the in vitro activities of cefditoren - a broad-spectrum oral cephalosporin - and other comparator agents against 2,396 fresh isolates from community-acquired respiratory tract infections, collected from 6 clinical Italian microbiology laboratories.

On penicillin-susceptible pneumococci and Streptococcus pyogenes, cefditoren demonstrated to be the most active antibiotic (MIC₉₀ values of 0.03 and 0.06 mg/L respectively), showing only a slight decrease in potency on penicillin-intermediate and re-sistant pneumococci (MIC₉₀ value 0.5 mg/L, 1.0 mg/L respectively). All the other comparators displayed MIC₉₀ values of 4 - 8 mg/L for penicillins and of 4 to >64 mg/L for the oral cephalosporins. Cefditoren and levofloxacin were the most active against MSSA (MIC₉₀ 0.5 mg/mL). Cefditoren displayed a uniformly potent inhibitory activity (MIC₉₀ of 0.03 mg/L) against all strains of Haemophilus influenzae, regardless of their ampicillin resistance (mediated or not by beta-lactamase production), while against Moraxella catarrhalis MIC₉₀ values were higher against beta-lactamase-positive (0.25 mg/L). Cefditoren was active also against Klebsiella pneumoniae and Escherichia coli: in this case its activity was comparable with that of levofloxacin.

In conclusion, cefditoren, due to its potent activity, is a new effective therapeutic option for the treatment of respiratory tract infections.     

In vitro activity of linezolid and other antibiotics against Gram-positive bacteria from the major teaching hospitals in Kuwait

Multidrug-resistant Gram-positive bacteria are an increasingly pressing problem in the clinic. Consequently, linezolid, a recently introduced oxazolidinone with Gram-positive activity, was tested in comparison with 10 other antibiotics against 8103 clinically significant Gram-positive cocci by Etest, disk diffusion and Vitek methods. Linezolid demonstrated excellent activities against all isolates. Vancomycin and teicoplanin demonstrated equally excellent activity against almost all isolates. The methicillin-resistant Staphylococcus aureus (MRSA) strains were all susceptible to the glycopeptides and linezolid, but resistant to erythromycin (96%), fusidic acid (91.5%), gentamicin (84%) and clindamycin (73%). Forty one and 39% of the Streptococcus pneumoniae isolates were resistant to penicillin (MIC >0.5 microg/ml) and erythromycin (MIC >1 microg/ml), respectively. S. agalactiae susceptibility was 9% and 10% resistant to clindamycin and erythromycin, respectively. In conclusion, all the Gram-positive isolates tested were susceptible to linezolid. With its oral bioavailability profiles, it obviously holds great promise. Our data should be a useful addition to the literature from the Middle East.     

Update of the Activity of Cefditoren and Comparator Oral β-lactam Agents Tested Against Community-Acquired Streptococcus pneumoniae Isolates (USA, 2004-2006)

Abstract

Cefditoren and other orally administered cephalosporins are infrequently included in resistance surveillance studies. Here we evaluated 359 contemporary (2004-2006) strains of Streptococcus pneumoniae, including penicillin-intermediate (12.0%) and -resistant (22.8%) subsets from United States patients by reference broth microdilution methods. Cefditoren was the most potent cephalosporin tested (MIC₅₀, 0.015 mg/L), including against penicillin-intermediate strains (MIC₅₀, 0.12 mg/L), and was two-, fourand eight-fold more active than cefuroxime, cefdinir and cefprozil, respectively. Penicillin- resistant strains were largely resistant to all tested β-lactams. We confirm the continued spectrum and potency for cefditoren against S. pneumoniae that surpasses that of other orally administered cephalosporins.     

Rationale for treating community-acquired lower respiratory tract infections with amoxicillin/sulbactam combination through pharmacodynamic analysis in the setting of aminopenicillin-resistant organisms

In order to establish a rationale for treating community-acquired lower respiratory tract infections, we assess here the pharmacodynamics of amoxicillin/sulbactam, 500mg/500mg, a formulation marketed in Argentina since 1988 and currently available in 17 countries, against the major pathogens, in comparison with that of a novel formulation (875mg/125mg, see J Chemother 2000; 12: 223-227). In time-kill studies, both bactericidal and inhibitory activity were seen in the 1.5- and 6-h sera, obtained from 12 volunteers after a single oral dose, against both a penicillin-susceptible and an -intermediate Streptococcus pneumoniae strain, as well as against Moraxella catarrhalis and a beta-lactamase-negative Haemophilus influenzae strain. Only the 1.5-h sera proved bactericidal against a penicillin-resistant S. pneumoniae strain (MIC, 2 microg/ml) and a beta-lactamse-positive H. influenzae isolate. This study suggests that amoxicillin/sulbactam (500mg/500mg) is still a suitable option for treating community-acquired lower respiratory tract infections, allowing a b.i.d. dosing schedule. Caution should be taken with pneumonia caused by beta-lactamase-positive H. influenzae or penicillin-resistant (MIC > or =2 microg/ml) S. pneumoniae isolates. Either shorter dosing intervals (t.i.d.) or a higher amoxicillin content in the formulation (i.e. 875 mg) may be required in these situations.     

Susceptibility of Respiratory and Urinary Pathogens to Ceftibuten and Other Comparative Drugs: Results of an Italian Multicenter Survey

Summary

A survey aimed at assessing the ability of ceftibuten, a new oral third-generation cephalosporin, to eradicate in Vitro selected bacterial pathogens was conducted in 1991 in 17 microbiology laboratories evenly distributed in Italy. Over 8700 organisms collected from in- and outpatients affected mainly by respiratory and urinary tract infections were analyzed. This collection of bacteria did not include staphylococci, enterococci, Pseudomonta and other oxidative species naturally refractory to the action of most antibiotics employed.

Susceptibility to ceftibuten, cefaclor, cefuroxime, amoxicillin, amoxicillin-clavulanate, cotrimoxazole and erythromycin was assessed using a standardized agar-diffusion method. Production of β-lactamases was confirmed by the nitrocefin test. Among the microorganisms studied E. coli (32.1%) prevailed, followed by P. mirabilis (17.1%), K. pneumoniae (10.9%), 5. pyogenes (6.6%), E. cloacae (5.1%), Serratia spp. (4.5%), Enterobacter spp. (4.2%), H. influenzae (3.6%), S. pneumoniae (2.2%) and Af. catarrhalis (2%). Within this group of pathogens amoxicillin resistance, often mediated by synthesis of P-lactamases, was widely diffused (46.2%). The overall inhibitory activity of the drugs tested decreased as follows: ceftibuten (90.4%), cefuroxime (80.4%), amoxicillin-clavulanate (77.4%), cotrimoxazole (75.3%), cefaclor (72.6%), amoxicillin (53.8%) and erythromycin (32.8%). When the efficacy of the antibiotics was assessed against the collection of respiratory isolates producing β-lactamases only ceftibuten maintained the same overall potency manifested against the general population while the comparative agents were far less effective. The results of this national survey indicate that, given the low incidence of resistance among the most prevalent causative agents of respiratory and urinary tract infections, ceftibuten can be safely used at present in the empiric therapy of these conditions especially when they occur in community settings.     

Comparison of Cefaclor and Cefuroxime Axetil in the Treatment of Acute Otitis Media with Effusion in Children Who Failed Amoxicillin Therapy

Abstract

This trial compared the efficacy and safety of a 10-day treatment course of cefaclor and cefuroxime axetil in the treatment of acute otitis media with effusion in children who failed therapy with amoxicillin.

This was an investigator-blind, randomized, parallel treatment group study. To be included, patients must have received treatment with a standard clinical regimen of amoxicillin for at least 48 hours and not more than 10 days, with the last dose within 72 hours of randomization. Patients who met the entry criteria were randomly assigned to one of two antibiotic treatment groups. Cefaclor and cefuroxime axetil suspensions were administered twice daily for a total daily dose of 40 mg/kg and 30 mg/kg, respectively. Physical examination, pneumatic otoscopy and tympanogram were performed to evaluate efficacy to therapy. Therapeutic equivalence was established by ruling out a difference (cefaclor minus cefuroxime axetil) of 15% in percentages of clinical success (cure plus improvement). Safety evaluation was performed by assessment of clinical adverse events.

In the intent-to-treat analysis post-therapy (1-6 days after completion of therapy), 96 of 104 (92.3%) cefaclor-treated patients had clinical success compared to 90 of 101 (89.1%) cefuroxime axetil patients. The 95% confidence limits on the difference between proportions of favorable outcomes (cefaclor minus cefuroxime axetil) was from ?4.8% to +11.2%. At termination of the study (day 10-16 after completion of therapy), 86 of 104 (82.7%) cefaclor patients and 84 of 101 (83.2%) cefuroxime axetil patients had favorable clinical outcomes (95% confidence interval: ?10.8% to +9.9%). Thirty-two (30.8%) of the 104 patients in the cefaclor treatment group reported at least one adverse event, with rhinitis reported in 9 (8.7%) patients and cough increased in 7 (6.7%) patients. Thirty-six (35.6%) of the 101 patients in the cefuroxime axetil treatment group reported at least one event, with diarrhea reported in 11 (10.9%) of patients and rhinitis in 10 (9.9%) patients.

Cefaclor and cefuroxime axetil were equally effective in the treatment of patients with acute otitis media with effusion who had failed therapy with amoxicillin. Significantly fewer patients treated with cefaclor reported diarrhea, which is the most frequently reported adverse event in children treated with antibiotics for this disease.     

Natural antibiotic susceptibility of Proteus spp., with special reference to P. mirabilis and P. penneri strains

The natural susceptibility of 102 Proteus mirabilis and 35 Proteus penneri strains to 71 antibiotics was examined. Minimum inhibitory concentrations (MICs) were determined by applying a microdilution procedure in IsoSensitest broth (for all strains) and cation-adjusted Mueller Hinton broth (for some strains). P. mirabilis and P. penneri were naturally resistant to penicillin G, oxacillin, all tested macrolides, lincosamides, streptogramins, glycopeptides, rifampicin and fusidic acid. Both species were uniformly, naturally sensitive to all tested aminoglycosides, acylureidopenicillins, some cephalosporins, carbapenems, aztreonam, quinolones, sulfamethoxazole and co-trimoxazole. Species-specific differences in natural susceptibility affecting clinical assessment criteria were seen with tetracyclines, several beta-lactams, chloramphenicol and nitrufurantoin. P. mirabilis was naturally resistant to all tested tetracyclines, and was naturally sensitive to all beta-lactams, except penicillin G and oxacillin. Strains of P. penneri were naturally sensitive or of intermediate susceptibility to tetracyclines, and naturally resistant to amoxicillin (but sensitive or of intermediate susceptibility to aminopenicillins in the presence of beta-lactamase inhibitors) and some cephalosporins (i.e. cefaclor, cefazoline, loracarbef, cefuroxime, cefotiam, and cefdinir). P. penneri was less susceptible than P. mirabilis to chloramphenicol; P. mirabilis was less susceptible than P. penneri to nitrofurantoin. Major medium-dependent influences on the MICs were seen with fosfomycin. The present study describes a database concerning the natural antibiotic susceptibility of P. mirabilis and P. penneri strains to a range of antibiotics, which can be applied to validate forthcoming antibiotic susceptibility tests of these bacteria. It was shown that ten of fifteen amoxicillin-sensitive P. mirabilis strains produced beta-lactamases at a low level, supporting the thesis of the presence of a naturally-occurring beta-lactamase in this species. Natural susceptibility patterns are compared with those of a recent study, dealing with natural susceptibilities of species of the P. vulgaris complex.