Currently one of the most common disease-causing bacterium in the world, Acinetobacter baumannii, for sure, is a nasty bug — an emerging nosocomial (hospital-associated) pathogen, being increasingly observed in serious conditions requiring intensive care (including ventilator-associated pneumonia, sepsis, meningitis, wound infection and urinary tract infection). Unfortunately for patients, particularly immune-suppressed ones, this bug is now known to be extensively drug resistant (XDR; resistant to most antibiotics including carbapenems, with the exception of two drugs of last resort, colistin and tigecycline), with a smaller proportion resistant to even these two (known as pan-drug resistant, PDR, which are therefore virtually untreatable with the current crop of FDA-approved medications).
Scientists, however, are not hung up on this Doomsday scenario, and are actively working to figure out ways to effectively counter this menace. It was in 2012 that I had written a post about an Acinetobacter therapeutics study emerging from the laboratory of Dr. Brad Spellberg (then at UCLA; currently at the University of Southern California). The aspect of the study that had fascinated me the most was the idea that the disease-causing bacterium Acinetobacter baumannii, as well as its more malevolent drug-resistant forms, could be stopped in their tracks by simply taking away the fangs they bite with; in other words, inhibiting the production of a crucial bacterial surface component —a lipopolysaccharide (LPS) that reacts strongly with host defence and induces the immune mediators to cause damage to the host tissue— could help destroy all forms of this pathogen, whether drug-resistant or not.
Earlier the same year, the Spellberg group had also identified a different therapeutic strategy against lethal XDR A. baumannii infection. Focusing on the fact that mice, after infection with this bug, produced large quantities of antibodies against components of the bacterial membrane, they figured out the major component responsible for most antibody reactivity, an outer cell membrane protein called OmpA — which in turn appeared to be a good vaccine antigen. This means:
- The OmpA protein was reasonably stably present in the bug.
- Immunizing naïve mice with recombinant OmpA resulted in generation of copious IgG antibodies.
- In both young and old mice, immunization with OmpA led to better survival and less bloodstream infection, when infected with a deadly dose of the bug.
- Analysis of antibody amount in the blood of mice showed its strong correlation to survival, i.e. more specific antibody = better survival.
But wait, that is not all! The protective power of the antibodies generated via direct immunization (‘vaccination’) with OmpA appeared to be retained in the immune blood, so that transferring the antibody-containing immune serum to naïve mice (a process known as ‘passive immunization’) was also effective in protecting the naïve mice from a subsequent lethal challenge by the bug. Further experiments with the immune serum revealed that the antibodies raised against OmpA could mobilize the host’s cellular defence mechanism to eat and destroy the bacterium, and remove it effectively from the tissues as well.
Further exploration from this group in 2013 has elucidated some more of the biological mechanisms associated with the OmpA-mediated protection; they found that a higher vaccine dose led to more antibodies, and the vaccine could engage both B-lymphocytes (which make antibodies) and T-lymphocytes (cells involved in adaptive immunity) — but it prefers those T-cells that are less involved in inflammatory processes. This anti-inflammatory mechanism (referred to as a ‘Type 2 immune response’) is likely beneficial, given that excessive inflammation (e.g. as is mediated by A. baumannii LPS) is damaging to the host.
Continued work is, of course, required, especially since encouraging results in a murine model do not always pan out in humans. The OmpA seems an excellent choice for a vaccine antigen, but there may be a very small (but extant) percentage of the bug strains which don’t express OmpA (which means that the anti-OmpA antibody may not recognize them). Nevertheless, the evidence suggests that vaccination with OmpA and/or treatment with an agent that can suppress LPS production by the bug may likely confer protection against a vast majority of A. baumannii strains, even if they have become resistant to conventional antibacterial drugs.
What prompted me to revisit my old post and look up Brad Spellberg’s work in this field? On Twitter, science communicator Maryn McKenna pointed at a report, published today, which made some serious observations regarding Carbepenem-resistant A. baumannii. For context, carbapenem-resistant Acinetobacter is found all over the world; genetic studies have grouped the clinical isolates into eight distinct clusters, of which a cluster called European Clone II is present across the Americas, Europe, Israel, Asia, Australia and South Africa. Interestingly, it is not normally associated with the skin or the environment; but we don’t know if it has any natural reservoir. What we do know is that it is extremely hardy, able to survive long-term on a variety of common fomites, which is possibly why it spreads via human contact transmission and/or environmental contamination.
The molecular signatures of these resistant bacteria are well-studied, so that scientists are able to match them to clinical and environmental isolates. The study published in Eurosurveillance today investigated A. baumannii isolates recovered from a secondary municipal waste-water treatment plant in Zagreb, Croatia, which receives combined sewage water from domestic, hospital, industrial and storm water sources; the bug was discovered in both the raw waste-water pouring into the plant (~60% in proportion in relation to all heterotrophic bacteria found) and the treated effluent (~30% similarly), and all the isolates were resistant to multiple drugs, including carbapenem. What’s more alarming is that some of these bugs could survive and multiply in the presumed-sterilized water coming out of the treatment plant. (Note: ‘heterotrophic bacteria’ are ones that can use organic substances for sustenance.)
As the research report points out, “In Croatia, proportions of carbapenem-resistant A. baumannii strains in clinical samples increased drastically from 2008 to 2012 (10% to 73% of isolates respectively), with some individual hospitals recording a rate of 90%” — which, unfortunately, fits well with the observations made in this paper. Based on the genetic signatures of these isolates, the authors have speculated if some of these mutation-carrying genes, responsible for carbapenem resistance, don’t also confer some resistance to adverse environmental conditions in sewage as well as the chemical disinfection (via chlorination) process.
Carbapenem resistance in A. baumannii is generally conferred by over-expression of a group of enzymes called β-lactamases, capable of breaking up a broad range of β-lactam antibiotics, including carbapenem; the β-lactamases active in A. baumannii are mostly oxacillinases, and less frequently, metallo-β-lactamases (MBLs). The bugs isolated from the Croatian plant contained both, including the dreaded New Delhi Metallo-β-Lactamase (NDM-1). This is of serious public health concern; these observations cast doubt on the efficacy of chlorination for sewage treatment, and more importantly, underline the grave realization that the effluent waste-water is adding this virulent pathogen to the Sava river, and finally into the Danube — whose eventual environmental and public health consequences may be grievous.