Tolerance of Haemophilus influenzae to beta-lactam antibiotics

Abstract

Two hundred clinical isolates of Haemophilus influenzae were tested for tolerance (MBC/MIC 2 32) to ampicillin and cefotaxime by broth dilution tests. Of 200 strains, 9 were tolerant to ampicillin, and 10 were tolerant to cefotaxime. Tolerant organisms were identified in both systemic and nonsystemic infections and among different biotypes and serotypes of If. influenzae. These tolerant isolates were compared with nontolerant isolates by broth dilution and killing curves with log-phase and stationary-phase inocula. Both tolerant and nontolerant bacteria in log phase were killed more rapidly by antibiotics than bacteria in stationary-phase growth. When tested against 11 different 3-lactams, several patterns of tolerance were observed. Six of the ten strains were tolerant to aztreonam, four were tolerant to cefuroxjme, three were tolerant to cefamandole, and two were tolerant to cefoxitin. Strain H130 was tolerant to all -3-lactam antibiotics studied. None of the 10 tolerant H. influenzae isolates were tolerant to chloramphenicol, rifampin, tobramycin, ciprofloxacin, enoxacin, and trimethoprim-sulfamethoxazole. Although the clinical significance of tolerance is not determined, this study suggests that the bactericidal activity (MBC) of ,B-lactam antibiotics against H. influenzae should be determined in cases of severe infections in which clinical response is slow or unsatisfactory.The most common mechanism of ampicillin resistance in Haemophilus influenzae is the production of plasmidmediated P-lactamase (13,20,22). In addition, there have been' several 'reports of P-lactamase-negative strains of H. influenzae resistant to , B-lactam (10, 17, 27). Altered cell membrane permeability and altered penicillin-binding proteins have been suggested as mechanisms of this resistance (14,19). In the past years, we have observed that H. influenzae may be also tolerant to P-lactam antibiotics (2, 25). Antibiotic tolerance is a form of resistance that occurs when the MIC of an antibiotic is low but the MBC is high and exceeds the MIC by at least 32 times (21). The incidence and characteristics of this phenomenon have been well described previously in gram-positive bacteria, particularly in Staphylococcus aureus and Streptococcus species (3,6,9,21). A deficiency of autolytic enzyme activity has been suggested as a possible mechanism of this resistance (28,29).In the' present investigation, we have evaluated the incidence of resistance and tolerance to ampicillin and cefotaxime among 200 clinical isolates of H. influenzae. Antibiotic susceptibility of tolerant strains was compared with that of nontolerant microorganisms by broth dilution and killing curve tests with log-phase and stationary-phase inocula. The effect of storage on the stability of antimicrobial agent tolerance was also studied. to detect tryptophanase (indole), urease, and ornithine decarboxylase. Pittman serotype (a through f or nonserotypable) was determined by direct slide agglutination with polyvalent and monovalent typing sera (Difco). All isolates were teste...

Mechanisms of beta-lactam resistance inHaemophilus influenzae

“…Tolerance has been demonstrated to exist in Haemophilus influenzae (42). As previously demonstrated for clinical isolates of pneumococci, tolerance can be associated with nonenzymatic resistance in about 50% of the tolerant…”

“…This phenomenon appears to occur specifically, but not necessarily for all beta-lactam antibiotics since among ten strains tolerant to cefotaxime, nine were tolerant to ampicillin, six to aztreonam, four to cefamandole, and two to cefoxitin (42). The molecular basis for tolerance in…”

“…In conjunction with the clinical isolates shown to be tolerant to cefotaxime, no obvious cases of clinical failure were reported (42).…”

Ampicillin killing curve patterns of Haemophilus influenzae type b isolates by agar dilution plate count method

“…Bergeron and Lavoie (1) described ampicillin tolerance (MBC/MIC, .32) for 9 of 165 ampicillin-susceptible strains tested as stationary-phase inoculum preparations. When the nine were subsequently tested as exponential-phase inoculum preparations, only four remained tolerant.…”

Transposon mutagenesis identifies genes which control antimicrobial drug tolerance in stationary-phaseEscherichia coli

“…Firstly, bacteria quickly become tolerant [2][3][4] to the agents. This is temporary, reversible, is associated with slowing of the rate of multiplication [5,2,6,4] and enables bacteria to survive a broad range of agents and other stress conditions such as starvation [7][8][9]. Secondly, bacteria can undergo genetic mutation which leads to permanent clonal resistance to antimicrobial agents.…”