The role of type VI secretion system genes in antibiotic resistance and virulence in Acinetobacter baumannii clinical isolates.

Introduction: The type VI secretion system (T6SS) is a crucial virulence factor in the nosocomial pathogen Acinetobacter baumannii. However, its association with drug resistance is less well known. Notably, the roles that different T6SS components play in the process of antimicrobial resistance, as well as in virulence, have not been systematically revealed. Methods: The importance of three representative T6SS core genes involved in the drug resistance and virulence of A. baumannii, namely, tssB, tssD (hcp), and tssM was elucidated. Results: A higher ratio of the three core genes was detected in drug-resistant strains than in susceptible strains among our 114 A. baumannii clinical isolates. Upon deletion of tssB in AB795639, increased antimicrobial resistance to cefuroxime and ceftriaxone was observed, alongside reduced resistance to gentamicin. The ΔtssD mutant showed decreased resistance to ciprofloxacin, norfloxacin, ofloxacin, tetracycline, and doxycycline, but increased resistance to tobramycin and streptomycin. The tssM-lacking mutant showed an increased sensitivity to ofloxacin, polymyxin B, and furazolidone. In addition, a significant reduction in biofilm formation was observed only with the ΔtssM mutant. Moreover, the ΔtssM strain, followed by the ΔtssD mutant, showed decreased survival in human serum, with attenuated competition with Escherichia coli and impaired lethality in Galleria mellonella. Discussion: The above results suggest that T6SS plays an important role, participating in the antibiotic resistance of A. baumannii, especially in terms of intrinsic resistance. Meanwhile, tssM and tssD contribute to bacterial virulence to a greater degree, with tssM being associated with greater importance.

The optimized carbapenem inactivation method for objective and accurate detection of carbapenemase-producing Acinetobacter baumannii.

The modified carbapenem inactivation method (mCIM) recommended by the Clinical and Laboratory Standards Institute is not applicable for detecting carbapenemases in Acinetobacter baumannii. Four currently reported phenotypic detection methods, namely, the modified Hodge test, the mCIM, the adjusted mCIM, and the simplified carbapenem inactivation method (sCIM), did not perform well in our 90 clinical A. baumannii isolates. Thus, the minimal inhibitory concentrations (MICs) of carbapenems and the existence and expression of carbapenemase-encoding genes were detected to explain the results. According to the E-test, which was more accurate than the VITEK 2 system, 80.0 and 41.1% were resistant to imipenem (IPM) and meropenem (MEM), respectively, and 14.4 and 53.3% exhibited intermediate resistance, respectively. Five ß-lactamase genes were found, of which blaOXA-51-like, blaTEM, and blaOXA-23-like were detected more frequently in 85 non-susceptible strains. The expression of blaOXA-23-like was positively correlated with the MIC values of IPM and MEM. Therefore, an improved approach based on the mCIM, designated the optimized CIM (oCIM), was developed in this study to detect carbapenemases more accurately and reproducibly. The condition was improved by evaluating the factors of A. baumannii inoculum, incubation broth volume, and MEM disk incubation time. Obvious high sensitivity (92.94%) and specificity (100.00%) were obtained using the oCIM, which was cost-effective and reproducible in routine laboratory work.

Probiotic Lactobacillus Species and Their Biosurfactants Eliminate Acinetobacter baumannii Biofilm in Various Manners.

Acinetobacter baumannii is a critical biofilm-forming pathogen that has presented great challenges in the clinic due to multidrug resistance. Thus, new methods of intervention are needed to control biofilm-associated infections. In this study, among three tested Lactobacillus species, Lactobacillus rhamnosus showed significant antimaturation and antiadherence effects against A. baumannii biofilm. Lactic acid (LA) and acetic acid (AA) were the most effective antibiofilm biosurfactants (BSs) produced by L. rhamnosus. This antibiofilm phenomenon produced by LA and AA was due to the strong bactericidal effect, which worked from very early time points, as determined by colony enumeration and confocal laser scanning microscope. The cell destruction of A. baumannii appeared in both the cell envelope and cytoplasm. A discontinuous cell envelope, the leakage of cell contents, and the increased extracellular activity of ATPase demonstrated the disruption of the cell membrane by LA and AA. These effects also demonstrated the occurrence of protein lysis. In addition, bacterial DNA interacted with and was damaged by LA and AA, resulting in significantly reduced expression of biofilm and DNA repair genes. The results highlight the possibility and importance of using probiotics in clinical prevention. Probiotics can be utilized as novel biocides to block and decrease biofilm formation and microbial contamination in medical equipment and during the treatment of infections. IMPORTANCE A. baumannii biofilm is a significant virulence factor that causes the biofilm colonization of invasive illnesses. Rising bacterial resistance to synthetic antimicrobials has prompted researchers to look at natural alternatives, such as probiotics and their derivatives. In this study, L. rhamnosus and its BSs (LA and AA) demonstrated remarkable antibiofilm and antimicrobial characteristics, with a significant inhibitory effect on A. baumannii. These effects were achieved by several mechanisms, including the disruption of the cell envelope membrane, protein lysis, reduced expression of biofilm-related genes, and destruction of bacterial DNA. The results provide support for the possibility of using probiotics and their derivatives in the clinical prevention and therapy of A. baumannii infections.

Uncovering the Secretion Systems of Acinetobacter baumannii: Structures and Functions in Pathogenicity and Antibiotic Resistance.

Infections led by Acinetobacter baumannii strains are of great concern in healthcare environments due to the strong ability of the bacteria to spread through different apparatuses and develop drug resistance. Severe diseases can be caused by A. baumannii in critically ill patients, but its biological process and mechanism are not well understood. Secretion systems have recently been demonstrated to be involved in the pathogenic process, and five types of secretion systems out of the currently known six from Gram-negative bacteria have been found in A. baumannii. They can promote the fitness and pathogenesis of the bacteria by releasing a variety of effectors. Additionally, antibiotic resistance is found to be related to some types of secretion systems. In this review, we describe the genetic and structural compositions of the five secretion systems that exist in Acinetobacter. In addition, the function and molecular mechanism of each secretion system are summarized to explain how they enable these critical pathogens to overcome eukaryotic hosts and prokaryotic competitors to cause diseases.

An Outer Membrane Protein YiaD Contributes to Adaptive Resistance of Meropenem in Acinetobacter baumannii.

Acinetobacter baumannii is an important nosocomial pathogen that can develop various resistance mechanisms to many antibiotics. However, little is known about how it evolves from an antibiotic sensitive to a resistant phenotype. In this study, we investigated the transition of outer membrane proteins (OMPs) under antibiotic stress and identified YiaD as an OMP marker involved in the development of adaptive resistance to meropenem (MEM) in A. baumannii. Following stimulation of a carbapenem-sensitive strain AB5116 with sub-MIC of MEM, yiaD showed significantly decreased expression, and this decrease continued with prolonged stimulation for 8 h. The downregulation of yiaD was not only observed in clinically sensitive strains but also in 45 carbapenem-resistant isolates that produced the ß-lactamases TEM and OXA-23. However, the extent of the reduction of yiaD expression in resistant strains was less than that in sensitive strains. Lack of yiaD resulted in a 4-fold increase in the MIC of AB5116 to MEM. The same level of depressed susceptibility induced by yiaD deletion was observed in both a growth curve test and a survival rate assay. Moreover, the colony shape became enlarged and irregular after loss of yiaD, and the biofilm formation ability of A. baumannii was influenced by YiaD. These results suggest that YiaD could respond to the stimulus of MEM in A. baumannii with a downregulation trend that kept pace with the prolonged stimulation time, indicating that it participates in various routes to benefit MEM resistance evolution in both carbapenem-sensitive and -resistant A. baumannii strains. IMPORTANCE Acinetobacter baumannii can develop various resistance mechanisms to carbapenems. However, the factors involved in the evolutionary process that leads from transition to the sensitive to resistant phenotype are not clear. The outer membrane protein YiaD of A. baumannii was downregulated under the stress of meropenem (MEM), and its expression level was continuously reduced with prolonged stimulation time. The downregulation of yiaD was not only observed in sensitive strains but also in carbapenem-resistant isolates producing the ß-lactamases TEM and OXA-23. However, the extent of yiaD reduction was less in resistant strains than in sensitive strains. Lack of yiaD resulted in an increased MEM MIC, enlarged and irregular colonies, and decreased biofilm formation ability. These results suggest that YiaD responds to MEM stimulus in A. baumannii and participates in the adaptive resistance of MEM in both carbapenem-sensitive and -resistant strains.

Phenotypic and genotypic characteristics of Acinetobacter baumannii enrolled in the relationship among antibiotic resistance, biofilm formation and motility.

Acinetobacter baumannii is an important pathogen in clinical. The factors of biofilm formation, antibiotic resistance and motility contribute great to A. baumannii in persisting in stressed environment, and further leads to nosocomial infections. 70 A. baumannii clinical isolates were investigated for their clinical characteristics of infection. Among the tested strains, 54 (77.1%) isolates were obtained from ICUs, with the frequency of multidrug-resistance (MDR) at 55.7%, and that of extensively drug-resistance (XDR) at 31.4%. 97.1% of the clinical isolates could form biofilms, in which 4.3% possessed weak biofilm formation ability, while 41.4% and 51.4% were moderate and strong biofilm producers, respectively. A strong correlation between antibiotic resistance and biofilm formation ability was found that all the resistant strains could form biofilms, with the majority in moderate and strong levels, but 2.9% sensitive isolates had no such ability. However, the sensitive strains that could produce biofilms showed stronger biofilm formation capacity in the early stage before 24 h compared to the resistant isolates, though they became weaker afterwards. 24 biofilm-related genes and two blaOXA genes were found in both biofilm-forming and non-biofilm-forming strains, but with higher prevalence in the strains that could produce biofilms. No correlation was detected between twitching motility with antibiotic susceptibility or biofilm formation. These results raised a viewpoint that examining timepoint is a key factor for determining the biofilm formation ability, and further highlighted the importance of the appropriate surveillance and control measures in preventing the emergence and transmission of MDR and XDR A. baumannii.