Necessary background: One major way that microbiologists classify bacteria is on the basis of the organisms’ cell membranes; some have a single membrane, and others have two separated by fluid. The groups are identified by their response to a 4-step staining process, called Gram stain for the Danish physician who invented it in the 1880s. Cells that pick up the first stain applied, which is usually violet but sometimes blue, are single-walled; cells that resist the bath of the first stain, but pick up a lighter tint from another chemical in a later step, are double-walled. The single-membrane, dark-stained organisms are dubbed Gram-positive; the double-membrane organisms are known as Gram-negative.
Here’s why that distinction is so important for understanding antibiotic resistance: Most of the drugs that kill or control bacteria act by attaching to or penetrating through cell membrane. The double membrane of the Gram-negatives presents a greater obstacle to drug-molecule interference than the single membrane of the Gram-positives — and thus makes developing drugs that can control Gram-negatives a more complex task. Hence, while there’s abundant concern about the narrowing drug pipeline for Gram-positives including MRSA, there is even more alarm about the dearth of new drugs for Gram-negatives (as captured last year in this article from Clinical Infectious Diseases).
The novel resistance factor that is described today in Lancet ID appears only in Gram-negatives, primarily in E. coli and K. pneumoniae but also in other species. Bacteria that have acquired this mechanism are resistant to multiple classes of drugs commonly used against Gram-negatives: beta-lactams, fluoroquinolones, aminoglycosides, and most troublingly carbapenems, generally considered the drug class of last resort for those organisms. Several of the isolates found in the study were susceptible only to colistin, a drug that dates back to the 1960s and is considered toxic to the kidneys, and tigecycline, which was only licensed in the US in 2005. Several responded only to aztreonam. One was susceptible to nothing.
The real threat in today’s news, though, is not only how resistant these organisms have become; it is also how they got that way, and how and by what means they are spreading.
As the Lancet ID paper reports, NDM-1 resides on a plasmid — a snippet of DNA, not on a chromosome, that reproduces on its own and can move freely between organisms. Intuitively, you would think that bacteria either inherit resistance from their progenitors or develop it on their own when they encounter a drug. Plasmids short-circuit both those processes, allowing resistance to spread rapidly within a single bacterial generation to organisms that have never experienced the drug they are acquiring defenses against. And as the paper testifies, NDM-1 has spread: The authors surveyed for NDM-1 in India, Pakistan and the UK, and found it both widely distributed in South Asia, and also present in UK residents who had family or business ties to South Asia, or had gone to the subcontinent for medical care. And unlike some resistant organisms, the bacteria carrying NDM-1 were not confined to the bug-friendly environment of hospitals or the the debilitated systems of hospital patients. Instead, it was out in the community, causing common illnesses such as urinary tract infections.
There are a couple of points embedded in that report that bear unpicking because they are so foreboding.
First, that this is happening in India, which not only harbors some of the world’s largest manufacturers of generics, but also (and possibly synergistically) has some of the world’s highest rates of antibiotic use. Some Indian researchers have been warning for years that the subcontinent is on the verge of a homebrewed crisis of drug resistance (Indian Journal of Bioscience, Indian Journal of Medical Microbiology, Indian Journal of Medical Ethics).
Second, that it is linked to medical care, and especially to medical tourism — which has become a booming international industry, not only for elective options such as cosmetic surgery, but because it offers an inexpensive way to perform major procedures that health systems might once have wanted to have done close to the patient’s home. A study covered last January by The Independent in London recommended shipping UK patients to India for care, suggesting it could save the beleaguered health service more than $200 million.
And third, that these isolates were found in community infections caused by common organisms such as E. coli. That testifies not only to their wide distribution, but also to how difficult it might be to conduct surveillance for their presence — or, put another way, how easily they could evade detection while they continue to spread. It is not likely that physicians are going to culture every UTI that comes their way, either in the resource-poor developing world or in the overstressed conditions of Western medicine.
One example of the importance of surveillance: That’s how NDM-1’s first appearance in the United States was detected, via three isolates from three states that were tested at the CDC’s national labs in the first half of this year. In a bulletin in June (the subject of my first post on NDM-1), the CDC urged clinicians to be alert for resistant infections in any patients who reported receiving medical care in India or Pakistan.
Unfortunately, given the drought of new drugs for Gram-negatives, surveillance may be the best bet for controlling or at least slowing NDM-1’s further spread. It’s the urgent recommendation of the author of a companion Lancet ID editorial, also published today (and who appears to have seen Canada’s first case):
The spread of these multiresistant bacteria merits very close monitoring and worldwide, internationally funded, multicentre surveillance studies, especially in countries that actively promote medical tourism. Patients who have had medical procedures in India should be actively screened for multiresistant bacteria before they receive medical care in their home country. …The consequences will be serious if family doctors have to treat infections caused by these multiresistant bacteria on a daily basis.
Kumarasamy KK, Toleman MA, Walsh TR et al. Emergence of a new antibiotic resistance mechanism in India, Pakistan, and the UK: a molecular, biological, and epidemiological study. The Lancet Infectious Diseases, early online publication, 11 August 2010doi:10.1016/S1473-3099(10)70143-2
Pitout JDD, The latest threat in the war on antimicrobial resistance. The Lancet Infectious Diseases, early online publication, 11 August 2010. doi:10.1016/S1473-3099(10)70168-7