Role of ISAba1 and ISAba125 in governing the expression of bla ADC in clinically relevant Acinetobacter baumannii strains resistant to cephalosporins

Abstract

Acinetobacter baumannii is a multi-resistant opportunistic nosocomial pathogen responsible for several outbreaks worldwide. It can cause several infections at various sites of the body. One of the main infections caused by this bacterium is ventilator-associated pneumonia in patients in intensive care units. Treating these infections is becoming difficult because of the high resistance to antimicrobial agents. This study compared the expression of the chromosomally encoded bla ADC gene in isolates having ISAba1, ISAba125 and no insertion upstream of the bla ADC gene in A. baumannii clinical isolates. It showed that the expression of bla ADC was six times greater when ISAba125 was present upstream of the gene in comparison with the constitutively expressed bla ADC gene with no insertion present upstream. The study indicated that ISAba125 has better promoters than ISAba1 and this is responsible for the overexpression of the bla ADC gene as they share considerable homology to the well-established Escherichia coli promoters. The "10 box of ISAba125 formed a fusion promoter with the "35 box of the bla ADC gene causing the bla ADC gene to be significantly overexpressed. The ability to upregulate the expression of bla ADC with the assistance of different insertion elements such as ISAba1 and ISAba125 has become an important factor in A. baumannii resistance to cephalosporins. INTRODUCTIONThe predominant mechanism of b-lactam resistance in Gram-negative bacteria is the synthesis of b-lactamases (Philippon et al., 2002). Following the introduction of highly stable cephalosporins, the carbapenems and monobactams, resistance initially appeared in organisms such as Enterobacter cloacae, Citrobacter freundii, Serratia marcescens and Pseudomonas aeruginosa, where mutations in the chromosome lead to the overproduction of the AmpC blactamase. This confers resistance to oxyimino-and 7-alphamethoxy-cephalosporins and monobactams (Sanders, 1987). Class C b-lactamases produced by Gram-negative bacteria can hydrolyse many b-lactam antibiotics including the cephamycins and oxyimino-cephalosporins (Philippon et al., 2002;Sanders, 1987;Caroff et al., 2000). These enzymes can also hydrolyse monobactams, such as aztreonam, to a lesser extent (Bush et al., 1995;Mammeri & Nordmann, 2007). Acinetobacter baumannii is a Gramnegative, non-fermenting coccobacillus, which is associated with serious nosocomial infections (Cisneros & Rodríguez-Baño, 2002;Levin et al., 2003). Antimicrobial treatment of such infections is difficult due to their multidrug resistance profile (Corvec et al., 2003). The AmpC enzyme from multiresistant A. baumannii was first sequenced from a Spanish isolate (Bou & Martínez-Beltrán, 2000). This constitutively expressed AmpC enzyme shares low similarity with Enterobacteriaceae AmpC cephalosporinases (Corvec et al., 2003). High-level expression of AmpC b-lactamase is a mechanism of chromosomally mediated resistance to cephalosporins in A. baumannii (Bou & Martínez-Beltrán, 2000). Transposable elements are important motors...

Simultaneous Identification of Multiple β-Lactamases in Acinetobacter baumannii in Relation to Carbapenem and Ceftazidime Resistance, Using Liquid Chromatography-Tandem Mass Spectrometry

“…S1 in the supplemental material), did not result in resistance to the tested carbapenems, indicating that OXA-71 has little activity against carbapenems. The isolates that overexpressed bla ADC-like were all resistant to ceftazidime, which is in agreement with previous work (8,17). In the three ceftazidimeresistant isolates in which no ADC-like protein was detected, other ␤-lactamases with known cephalosporinase activity were identified, i.e., CMY-2-like in isolate 5, PER-1-like in isolate 10, and GES-1-like in isolate 30 (Table 1), which is in accordance with the detection of bla CMY-30 , bla PER-1 , and bla GES-11 , respectively, by PCR.…”

Relative Strengths of Promoters Provided by Common Mobile Genetic Elements Associated with Resistance Gene Expression in Gram-Negative Bacteria

“…Differences in replication and transcription machinery (19), codon preferences, and tRNA availability (20) may result in different levels of expression from identical promoters in different species. Expression of an Acinetobacter baumannii bla ADC (ampC) gene (determined by qRT-PCR) was reported to be ϳ2-fold higher from ISAba125 than from ISAba1 (21), and GFP expression from ISAba1 and ISEcp1 P OUT was significantly lower in K. pneumoniae ATCC 13883 than in E. coli DH5␣ in our own experiments, although these differences were less striking than the differential effects of gene interposition. P C promoters are found in a distinctive genetic context, usually with a tandem array of cassette-borne genes (four in pJIBE401 from clinical isolate Kp1239) that rely on them for expression, while IS promoters such as ISAba1 P OUT are typically found upstream of a single gene that requires a shorter transcript for expression.…”

Transposition of Tn 125 Encoding the NDM-1 Carbapenemase in Acinetobacter baumannii

“…Here, we experimentally demonstrated the functionality of ISAba125 (IS30 family). Consistent with its ability to transpose, ISAba125 was described in association with increased resistance levels to ␤-lactams in Acinetobacter spp., for example, through duplication of the bla OXA-58 carbapenemase gene (26), disruption of the carO gene encoding an outer membrane protein acting in synergy with other resistance mechanisms (27), or insertion upstream of the bla ADC cephalosporinase gene (28). In the sequenced genomes of strains listed in Table 3, ISAba125 was found upstream of the ampC gene in strain ACICU (29) or flanking the aphA6 gene conferring resistance to amikacin in strain BJAB0715 (30).…”