*For medical professionals only
Carbapenem antibiotics are known as the "last line of defense" against bacterial infections due to their wide antibacterial spectrum and strong activity.
However, since 2009, broad-spectrum carbapenem-resistant bacteria, driven by the metallo-β-lactamase (NDM)-encoding gene (blaNDM) in New Delhi [1], have spread widely worldwide.
In 2019 alone, carbapenem-resistant pathogens, such as Acinetobacter baumannii and Klebsiella pneumoniae, were responsible for 5 to 100,000 deaths worldwide [2]. However, until now, scientists have known little about the horizontal transfer of such resistance genes in complex bacterial communities.
Recently, a research team led by Zhou Shungui of Fujian Agriculture and Forestry University and Timothy R Walsh of Oxford University published a blockbuster research result in the prestigious journal The Lancet Microbiology [3].
They found that plasmids (pX3_NDM-5) carrying the NDM-5 coding gene have a wide host range and can be transferred between up to 16 bacterial phyla, rather than the narrow host range previously thought. They also confirmed that gram-negative bacteria can transfect pX3_NDM-5 to gram-positive bacteria, and that this plasmid can also be transfected back into gram-negative bacteria. In addition, Enterococcus faecalis can act as a natural shuttle vector for the widespread spread of pX3_NDM-5 plasmids.
It is understood that this study not only confirms for the first time that the pX3_NDM-5 plasmid is a wide-host plasmid, but also challenges the traditional notion that gram-negative bacteria and gram-positive bacteria rarely exchange clinically important plasmids. The first author of the paper is Yang Qiu'e from Fujian Agriculture and Forestry University.
Screenshot of the first page of the paper
In order to study the propagation status of the antimicrobial plasmid pX3_NDM-5 in the natural bacterial community, Zhou's team developed a dual fluorescent labeling method, namely labeling pX3_NDM-5 with green fluorescent protein (GFP) and donor E. coli (gram negative) with red fluorescent protein (mCherry). In this way, as long as pX3_NDM-5 enters the body of any bacteria, which bacteria will fluoresce green.
Subsequently, Zhou's team isolated 86 species of bacteria, including 39 gram-positive bacteria and 47 gram-negative bacteria, from the sewage treatment plant of Fujian Provincial Maternal and Child Health Hospital. Then, in the absence or presence of NaClO, E. coli carrying pX3_NDM-5 was co-cultured with the isolated bacteria.
After overnight co-culture, a large number of bacteria obtained pX3_NDM-5 plasmids, with green fluorescence occurring in a rate of 2.34% to 4.89%. The researchers believe that, contrary to previous beliefs, pX3_NDM-5 does spread quickly and widely in the bacterial community.
pX3_NDM-5 has escaped into other bacteria
So how exactly did the pX3_NDM-5 plasmid, which carries the carbapenem-resistant gene, be transferred from E. coli to those bacteria?
Gene sequencing results showed that the bacteria carrying the pX3_NDM-5 plasmid were distributed in 16 phyla, including gram-positive bacteria and some difficult-to-culture bacteria. Among them, the pX3_NDM-5 plasmid can be stable in three Enterococcus faecalis for up to 15 days. This discovery upends researchers' perception of such plasmids, which have traditionally had a very narrow range of hosts.
It is also worth noting that NaClO treatment increases the relative abundance of gram-positive bacteria carrying pX3_NDM-5 plasmids compared to culture environments without NaClO. This seems to imply that the commonly used disinfectant NaClO may promote the spread of drug-resistant plasmids pX3_NDM-5.
At the end of the study, Zhou's team also tested whether gram-negative bacteria (E. coli) could "retrieve" drug-resistant plasmids from gram-positive bacteria. They selected 40 species of bacteria across 6 genera that received pX3_NDM-5 plasmids from E. coli, and then co-cultured these bacteria with E. coli, and found that 35 of them were able to return drug-resistant plasmids to E. coli, with Enterococcus faecalis being one of the most frequently metastatic bacteria.
Research flowcharts
Overall, Zhou's team found that gram-negative bacteria can quickly and widely transmit important drug-resistant plasmids to gram-positive bacteria under simulated natural environmental conditions, which also challenges the traditional theory that there is little exchange of clinically important plasmids between gram-negative bacteria and gram-positive bacteria.
In any case, since there is a "reservoir" of carbapenem-resistant plasmids in nature, the "last line of defense" against bacterial infections will be impossible to hold sooner or later.
It is expected that scientists will further reveal the molecular mechanisms behind it and find ways to block the spread of drug-resistant plasmids.
Bibliography:
[1]. Yong D, Toleman MA, Giske CG, et al. Characterization of a new metallo-beta-lactamase gene, bla(NDM-1), and a novel erythromycin esterase gene carried on a unique genetic structure in Klebsiella pneumoniae sequence type 14 from India. Antimicrob Agents Chemother. 2009; 53(12):5046-5054. doi:10.1128/AAC.00774-09
[2]. Antimicrobial Resistance Collaborators. Global burden of bacterial antimicrobial resistance in 2019: a systematic analysis. Lancet. 2022; 399(10325):629-655. doi:10.1016/S0140-6736(21)02724-0
[3]. Yang QE, Ma X, Zeng L, et al. Interphylum dissemination of NDM-5-positive plasmids in hospital wastewater from Fuzhou, China: a single-centre, culture-independent, plasmid transmission study. Lancet Microbe. 2023. doi:10.1016/S2666-5247(23)00227-6
This article was written by BioTalker