Tag: antibiotics

No action today, no antibiotics tomorrow…

YOU may have missed the fact that today was a World Health Day devoted to antibiotics; if you hadn’t, then it is, or at least was. In any case, it’s more or less over now and the issue can sink into the din of background noise.

As Frank Swain put it in in his well researched, and typically pithy, Guardian article today:

Health experts have been ringing the alarm over antimicrobial resistance for so long that it seems to have become part of our collective background noise, like the endless rasp of waves on the shore. And like stupid tourists, we sleep in the sun while the tide comes in.

A little pithiness is warranted, because if we find ourselves still in this situation in 2021, I’m going to be either, a) A disgruntled cash-strapped senior lecturer / reader / professor with a serious Cassandra complex; b) long since departed from research due to lack of funding; or c) dead, or missing a limb, due to an untreatable bacterial infection, or grieving over the same in a loved one.

I’ve written previously about some of the reasons we don’t have new drugs, and we can keep re-stating these issues ad nauseam, but it doesn’t mean anything will actually change. The broad response of governments following the ReAct meeting in Stockholm last year was more words, then an eerie silence. Similarly, in a meeting of the British Society for Antimicrobial Chemotherapy (BSAC), bylined ‘The Urgent Need’, more words were said amongst people who already familiar with those words, following which there has also been an eerie silence.

Continue reading “No action today, no antibiotics tomorrow…” →

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Surviving antibiotics…

Killing avoidance strategies

A couple of recent research papers remind me that I promised to talk a little about a phenomenon by which bacteria can avoid being killed by antibiotics, without actually being resistant in the classical sense, i.e. they can’t actually grow in elevated concentrations of the antibiotics they survive, and those cells that do survive give rise to populations that are no more, or less, likely to survive next time.

The first paper comes from the lab of Prof. Tony Coates at the Centre for Infection at St. George’s, University of London. Prof. Coates has for a long time been heavily involved in research into the treatment of latent and persistent infections, most notably T.B./tuberculosis. His research team (as indeed are mine) are trying to understand why some antibiotics that kill actively growing bacteria of a particular species have no effect on cells of the same species that aren’t actively growing; almost akin to the bit in Jurassic Park where T. rex kills the lawyer who’s running, but wouldn’t have done had the lawyer stood still §.

One of the reasons for this is that, historically, most drug discovery has been focussed on targeting actively growing cells, but what we are increasingly finding is that persistent infections can be mediated by a recalcitrant population of slow-growing or non-growing cells.

Whilst the idea of targeting non-growing bacteria is not a wholly new idea (you can find a review on the subject by Prof. Coates in my ‘Further reading’), it does seem that together with the report’s first author, Dr Yanmin Hu, their spin-out company (Helperby Therapeutics) has developed a platform and proof of principle drug that is now in trials, demonstrating the utility of such an approach. They have identified an antibiotic compound that has potent anti-Staphylococcal activity, but importantly, acts specifically against non-multiplying cells.

In a second paper, brought to my attention by Ed Yong, the Collins lab in Boston has identified that a sub-population of super-resistant bacteria act in a charitable manner to other members of the colony that are less resistant. Whilst the super-resistant cells could satisfy their own selfishness by merely allowing all their less-resistant siblings to die out, the bacteria in this case have a mutation that maintains their production of indole (a signalling molecule) when normally its production would be shut down on exposure to the antibiotic. When released by the cell, indole stimulates non-resistant cells to enter a state of phenotypic resistance or ‘antibiotic survival’, even though continued-production of indole incurs a fitness cost.

Why might they do this?

Well, for one of the reasons that is very much the subject of my blog, bacterial fitness. As I have mentioned before, antibiotic resistance can have a fitness cost, which means that cells committing themselves to this ‘path of resistance’ may find themselves at a disadvantage come the time when the antibiotic is no longer around. The subject of my research is to document the various ways in which antibiotic resistant Staph. aureus mitigate these fitness costs so that they get to remain resistant and just as competitive as they ever were in the absence of antibiotic. It seems that in the case of the Collins’ lab’s charitable bacteria, they may mitigate the fitness cost of antibiotic resistance at a population level by maintaining the presence of non-resistant cells that can come to the fore once the antibiotic is removed.

“These few drug-resistant mutants, by enhancing the survival capacity of the overall population in stressful environments, may also help to preserve the potential for the population to return to its genetic origins should the stress prove transient. Efforts to monitor and combat antibiotic resistance are complicated by these bet-hedging survival strategies and other forms of bacterial cooperation.”

So what I want to do is briefly introduce the types of ‘antibiotic survival’ strategies seen in bacteria. It goes without saying that future drug discovery that targets the molecular/physiological underpinning for these strategies (once we’ve identified what these are!) will be important for the clinical management of infection.

Resistance or ‘killing avoidance’?

I’ve discussed in a previous post what I might describe as mechanisms of antibiotic resistance, i.e. producing a enzyme that modifies or chews up the antibiotic; or changing the component of the cell so that the antibiotic targeted to that component no longer has any effect, or pumping the antibiotic out of the cell before it does any damage.

It was recognised early on, in the heyday of antibiotics, that penicillin could kill most bacteria in a culture, but could not sterilise a culture. This has been observed with numerous antibiotic compounds, thus at a practical level you cannot achieve a 100% kill with antibiotics. Now this isn’t generally a problem for a healthy individual, as it is at this point the immune system takes over and clears away the remaining cells. However, many people receiving antibiotics aren’t well, they may be immuno-compromised, or suffering from a deep-seated infection. The persistence of a bacterial infection becomes a perfect breeding ground for classical antibiotic resistance, with each resurgence of the infection from surviving cells increasing the probability that resistance may evolve; and thus is thought to play a significant role in the failure of antibacterial treatment.

1. INDIFFERENCE. Bacteria can avoid being killed by being in a stationary phase (non-growing or metabolically inactive). This is actually the default repose of bacteria in the environment, only submitting to bursts of growth in the presence of nutrients. Most current (and old) antibiotics are specific to the particular cell components and processes of actively growing cells, there is no reason to expect that such antibiotics would have any killing effect on cells not engaging in these processes.

2. TOLERANCE. Those antibiotics that do kill bacteria don’t necessarily do so directly; they initiate a series of events, a cascade of physiological responses, which ultimately result in cell death. Unlike indifference, tolerance is not linked to the growth/metabolic state of the bacteria, but instead result from genetic changes that uncouple the killing activity of the drug from its inhibitory activity. In the clinic, tolerance seems to be specific to certain bacteria, and even then only in response to particualr antibiotics targeting the bacteria cell-wall.

3. PERSISTENCE. In a bacterial population there exists a sub-population of ‘persister’, cells that regardless of the growth state of the population as a whole, continue to exist in a stationary or growth-retarded state. It may be that persisters avoid antibiotic killing in the same way that indifferent bacteria do, but whilst there are some antibiotics that can kill indifferent cells, they don’t kill persisters; this suggests that something different is going on in these cells, and there is increasing evidence to suggest that there are defined genetic differences implicated in persistence, including changes within the stress-response pathways, but what these are (and what they do exactly) remains to be seen.

4. BIOFILMS. Finally, and most stubbornly, there is the issue of biofilms. Biofilms are like a condominium (or halls of residence) of bacteria, a structured environment where the bugs are surrounded by a gelatinous matrix of sugar chains and many other macromolecules. They are involved in some 80% of human infections and represent a major cause of antibiotic treatment failure. Within the matrix the bacteria avoid antibiotic killing through indifference and persistence, thought to be brought on by the low oxygen and low nutrient environment; the matrix also provides some protection from certain classes of antibiotics, as well as the immune system. Even if a large number of matrix surface cells are killed off, the matrix structure survives and can be re-populated by the surviving cells. For some bacteria the biofilm environment stimulates them to massively increase their rate of mutation, which can increase the rate at which antibiotic resistance can evolve.

So what do we do?

Well again it comes down to idealism versus pragmatism. The current system of drug discovery is fraught and inefficient enough without an additional burden of esoteric and poorly understood mechanisms of bacterial antibiotic survival. I do think there is some merit in drug discovery targeted at non-growing indifferent bacteria, this is particularly important in the treatment of T.B. The problem is going to be that many of these killing avoidance strategies differ between pathogens and between the particular environment in which they’re found, and also that in the absence of any ongoing preventative treatment, such as potential vaccines, by the time an infection manifests itself the antibiotic survival systems are likely to already be in place.

Other than indifference, biofilms are a system worth addressing in the immediate term. We have amassed a huge amount of data on biofilms, and demonstrated that they are of great clinical importance, thus efforts should be made to increase the number of biofilm busting compounds we have available.

Many people are familiar with antibiotic resistance, but I’m interested to hear (especially from other biologists) how much people knew about such antibiotic survival strategies. Also, as ever, please feel free to ask questions at any level. This (rather long) post barely touches the surface of this subject, there’s plenty more to be said!

^§^ The theory that T. rex would only ‘see’ moving objects is probably a little outdated.

Further reading

As always I will try to find open access material where available, and will update those references that aren’t as and when they do.

_Hu et al. (2010) A New Approach for the Discovery of Antibiotics by Targeting Non-Multiplying Bacteria: A Novel Topical Antibiotic for Staphylococcal Infections. PLoS ONE 5: e11818._
[ Open access ]

_Coates, A. et al. (2002) The future challenge facing the development of new antimicrobial drugs. Nature Reviews Drug Discovery 1: 895-910._
[ Free pdf ]

_Lee, H. et al. (2010) Bacterial charity work leads to population-wide resistance. Nature 467, 82-85._
[Sorry, article behind a paywall] – You can read Ed Yong’s post on it though.

Levin, B.R. and Rozen, D.E. (2006) Non-inhertied antibiotic resistance. Nat Rev Microbiol 4: 556-562.
[ free pdf ]

– A very useful grounding to the subject of phenotypic resistance, as it was understood back in 2006.

Lewis, K. (2010) Persister cells. Annu. Rev. Microbiol. 64: 357-72.
[Sorry, another paywall paper ]

– Good review of bacterial persistence.

[This post was restored from a WayBackWhen archive. It was originally posted to a blog called ‘The Gene Gym” that began life on the Nature Network in 2010, and then moved to Spekrum’s SciLogs platform.]

Compensating for alien genes…

[This post was restored from a WayBackWhen archive of an older incarnation of mentalindigestions.net]

“FROM the perspective of a bacterium, higher eukaryotes are oversexed, unadventurous and reproduce in an inconvenient way.” So says Pål Johnsen and Bruce Levin in their commentary of today’s article for discussion, and nary a truer word said. Of course, one may state that inconvenient as reproduction may be, bacteria clearly have no sense of fun.

There was once an idea that we could address the problem of antibiotic resistant bacterial strains by removing the ailing antibiotic from clinical use. In the absence of selective pressure it was thought that the evolutionary traits that enable the strain to resist the antibiotic would actually put the strain at a competitive disadvantage compared with a strain that doesn’t have such antibiotic resistance. The proposed cause of this? Fitness costs – these are imposed by a resource-expensive set of mutations, or carriage of alien DNA, that make the resistant strain compete less well once its non-resistant brethren are no longer being killed off by the antibiotic.

However, some years ago now experimental evidence suggested that this is not always the case; it may in fact be often not the case. It is worth mentioning at this point that it has been shown that in some circumstances (alt) the number of infections caused by a particular antibiotic-resistant pathogenic bacterium have become fewer on reduction (or removal) of the antibiotic to which that strain is resistant, but to assume this would be the case with all strains/antibiotics is naïve.

It is true, with few exceptions, that initially both plasmid and chromosomally encoded resistances result in fitness losses. However, when resistance has a cost it is possible for compensatory mutations in a cell to ameliorate these costs, usually without the loss of resistance. The type of compensatory mutations that mitigate the fitness cost of acquiring antibiotic resistance, or any other incoming DNA that encodes potentially useful genes, will depend very much upon the environment in which the bacteria finds itself. These include the availability of resources, i.e. the growth environment of the bacteria, the environment of the genes (mobile or chromosomal), or whether the genes are being selected for by an external factor, such as the presence of antibiotics in the case of resistance genes.

So what sort of ‘nips and tucks’ might a bacterial population undergo in order to maintain a battery of costly genes, but that may provide an ongoing advantage? Well, this is the subject of much ongoing research; one example indicated that, in the absence of selective pressure, costly genes are simply silenced – a molecular mechanism often found in higher organisms that prevents a gene from being ‘switched on’. Thus a reservoir of drug resistance determinants may remain in populations that have compensated for their presence, remaining ‘inactive’ until a selective pressure removes the silencing.

A recent study by Peter Lind (et al.), a grad student working in the lab of Dan Andersson at Uppsala, Sweden, addresses a particularly pertinent question of compensatory mutations: those associated with genes acquired by horizontal gene transfer (HGT). HGT bypasses the slow and haphazard process of evolution (via random mutation, selection and recombination) by offering an opportunity for bacteria to receive fully fledged genes encoding pathogenicity factors (genes that make bacteria better at causing disease) as well as genes that encode resistances to disinfectants and/or antibiotics, amongst others. There is no doubt that such incoming DNA may pose significant fitness costs, so Lind et. al. set out to quantify the nature of compensatory mutations on such incoming DNA.

Continue reading “Compensating for alien genes…” →

A radical source of antibiotic resistance…

A FEW years ago, a Boston University team headed by Jim Collins published findings that suggested the means by which bactericidal antibiotics result in cell death. Rather than the cause being the cellular target of the drug, the team showed it was the secondary effects of stimulating the production of hydroxyl radicals, a reactive oxygen species 1. The hydroxyl radical is known to cause significant damage to cellular DNA, proteins and cell wall, leading to cell death.

Their 2007 study 1 was initially met with a few raised eyebrows in some quarters, coming in for some criticism for having a few gaps; namely whether the role of the hydroxyl radical was even pertinent in a real world infections settings, which are often in the low-oxygen environment of biofilms 2. There was also some question of whether it was adequately demonstrated that the oxidative stress was a source or the result of cell damage. However, subsequent studies reported by Kohanski, as well as other labs, have described a more defined link between a bactericidal drug and resulting hydroxyl radical formation 3.

In the latest edition of Molecular Cell, a new article from Mike Kohanski, Mark DePristo and Jim Collins reports that prolonged exposure to sub-lethal concentrations of antibiotics can induce multiple drug resistance in E. coli and Staphylococcus aureus strains that were initially drug sensitive 4. E. coli strains were tested with sub-lethal levels of  three major classes of bactericidal antibiotics (quinolone, B-lactam and aminoglycoside), which were found to significantly increase the mutation rate, confirming their expectations.

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Targeting antibiotics…

rb1INHERITANCE, the process by which some of your parents DNA is repackaged in the agreeable form of you, can be described as ‘vertical gene transfer’, i.e. the passage of information down a lineage. However, this is not the only means by which DNA information can travel.

I once spent six years conducting research into the mechanisms by which resistance to antibiotics can be spread within, and between, bacterial species. Much of this focussed on horizontal gene transfer (HGT), specifically the transfer between bacteria of DNA packages called ‘plasmids’, which can contain a full set of instructions on how to resist an antibiotic. Unlike inheritance, HGT is more akin to you reaching out and placing your hand on your cousin and acquiring their ginger hair, or nose shape.

This is of course a very serious issue, in fact it has never been more serious. The subject of HGT is a key topic in many aspects of biological sciences, and I’ve blogged about some of the interesting aspects of such DNA information transfer before.

In the past 10 years or so, an oft’ discussed topic of conversation at the scientific conferences I’ve attended has been the development of targeted antimicrobials. This is a move towards being able to ‘take-out’ (in the mafia sense) those specific bacterial species that are causing a particular infection/disease, but without providing a selective pressure to develop resistance to the drug on this, and neighbouring, bacterial species.

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The perils of positivity….

IN science and medical publishing, everything is positive. Less than 4% of articles deal with negative results. There is a perception that negative results are non-results; only positive results are worth publishing. Why is it that showing that something does something is so much more important that showing that something doesn’t do something?

Obviously, I expect some common sense in this; I don’t very well expect that a paper should be published just because you have demonstrated that drinking water doesn’t cause sunburn, this would be a deeply unsurprising discovery. But what if it is a study that demonstrates that a particular drug doesn’t do what people expected it to do? What if it is a biotechnology that doesn’t work for a whole swathe of biological research?

Online science forums (or fora) are replete with anecdotal evidence describing how time, and time, and time again research scientists make the same mistakes, or encounter the same limitations, in particular techniques. This is because no-one ever publishes such limitations, or at least, not more than 4% of the time.

So what is the problem? Well, science is expensive. Very expensive. It is expensive in material cost, and it is expensive in research hours. To have discovered that you’ve wasted a year doing work that elsewhere in the world someone once wasted a similar amount of time doing, only, 3 years ago, is deeply frustrating.

In coffee breaks around the world, many scientists have discussed the idea of a Journal of Negative Results, a compendium that can be consulted at the outset of a research project to determine whether a technique or approach has already been taken toward a research problem, but has been found not to work. Sometimes such negative results a mentioned, but only in passing, and only after an alternative technique resulted in positive results, which resulted in the subsequent publication. They are rarely keyword searchable and thus inordinately difficult to find.

As I mentioned, science costs a lot of money, far more money than is necessary. This is largely because the money isn’t real, there is poor ownership of it, it is monopoly money. If it were coming out of our own pockets, we simply wouldn’t pay the price we do, we’d demand more competitive prices. Consumable companies are free to charge extortionate prices for items that they are producing by the million. I have tubes in my lab that cost £3.75 each; they can only be used once, and invariably one or two of them can be wasted due to one problem or another. Kits are all the rage in research; pre-fabricated methodologies with all the reagents and instructions one needs to perform a particular experiment. The reagents themselves cost practically nothing in most cases, yet the kits can cost anywhere between £300 – £1500, and in many circumstances, afford you between 5 – 20 experiments.

Now this combination of expensive research is part of what leads to negative results being unwanted. There’s no real money in debunking an idea, it must come along side a positive result if it is to come at all. In the pharmaceutical industry, it is part of the reason why any new drug being produced is just too much of an investment to allow to fail, so the pressure is on to ensure, by hook or by crook, that the drug is licensed. Ben Goldacre writes at length about this in his recent book, and blog of the same name, Bad Science; this is most definitely worth a read!

Expensive research also prevents investment into rarer diseases, or any medications that run the risk of having a short shelf-life. One class of drugs that have fallen foul of this economic equation are antibiotics, and this is a rather long pre-amble into what I wanted to say in this blog essay (or blessay, and Stephen Fry attests to horribly calling it).

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