Moselio Schaechter

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April 12, 2012

Talmudic Question #86

Given that so many kinds of bacteria are intimately associated with animals and plants, why are so relatively few pathogenic?


"More would I think if it were not for the immune response of a potential host organism. This relates to the above question as I guess this requires a lot of energy/specialised traits. However, viruses - if anything - put the increase of energy use on to the host and not themselves. So this shouldn't be an issue for viruses."

are there any non-pathogenic viruses?

That is essentially the same questions as why not all the algae in the pond are volvox. Just a matter of an evolutionary trade-off as our stupid politicians actually shows... You can not be the best player in everything. In the tug-of-war between bacteria and protozoa, sheltering, camouflage, changing shape of getting slippery is probably more effective for the former ones...

A little Lewis Thomas anybody? I write this quote on the inside cover of my lab notebooks, and I have done so since my undergrad days =)

"…even in our worst circumstances we have always been a relatively
minor interest of the vast microbial world. Pathogenicity is not the
rule. Indeed, it occurs so infrequently and involves such a relatively
small number of species, considering the huge population of
bacteria on earth, that it has a freakish aspect. Disease usually
results from inconclusive negotiations for symbiosis, an
overstepping of the line by one side or the other, a biological
misinterpretation of borders.”

Lewis Thomas 1974

Good responses so far, I would guess that its more of a competition between microbes for an ecological niche which ultimately "forces" some towards pathogenic lifestyles in order to survive. I think an example of this is H.pylori infection of the stomach where a "difficult" ecological niche is inhabited by a pathogenic bacteria. I can see similar parallels in pathogenic Staphylococcus species which end up in a highly competitive niche with commensal skin bacteria.

If Adam Smith ideas would govern nature, the cell scape would have been impossible and we would still be arrested in the RNA world. Fortunately, as John Nash stated in the bar scene: "...the best result will come...from everyone in the group doing what's best for himself...and the group". Social cheating has clear antrophocentric connnotations but it is not the way in which evolution works at long scales...

Good, good... How nice it is to realize again and again that when a bunch of minds focuses thinking on the same subject, a wonderful spectrum of thoughts originating from different perspectives allows seeing challenging questions in ways that beautifully complement each other and offer to the thinker with an open mind unexpected ways of looking at the problem… Thanks to all.

Today’s Talmudic question makes me remember the old controversy about the bdellovibrios: Were indeed parasites or predators of the host bacteria they eventually used as substrate to grow and multiply? Since the bdellovibrios discovery by H. Stolp and the pioneering (1970-1980) work in the subject by M. Starr, M. Shilo, S. Rittenberg, R. Seidler and a few others, that question has been going around and surfacing now and then in the discussions.

Now this one goes to Marvine who asked: Does God ever answer?
Hummm… May be He answers using the “language of the facts themselves” so we may look at them, touch them, smell the, hear them, eventually “taste them”, and, above all, think about them in myriad ways… (I am not sure if the last paragraph will pass censorship.I'll understand it doesn't...).

Agree with Vincent Racaniello and Paula--why spend a whole lot of energy fighting when you can live in peace with your host by being useful? Eat, sleep and lounge around on your fimbrae and secrete a toxic chemical against your gracious host's enemies every so often. Or make a yummy nutrient. Now I'm not shifting human values onto microbes --or am I? Hummm. Maybe they've shifted microbial values onto us.

An answer from evlutionary game theory can be found in Nowak's paper
Five rules for the evolution of cooperation, and related work. I expect that non-pathogeny, simply getting along with your host without killing it, should be even easier to evolve, so I'm not surprised it did. I would instead argue that pathogeny could be a transient state in the co-evolution of bacterias and their hosts, which ends in either the extinction of one of the species involved, or non-pathogeny.

I love these Talmudic Questions! I'm not good at writing them, but I truly do enjoy reading them and sharing them with students as "thought questions." This one is a doozy, particularly given the near automatic reflex among students that "germ" equals "pathogen."

For what it is worth, I think folks have given some interesting and thoughtful answers to Elio's Talmudic Question here. It is very clear to me that, for both pathogens and mutualists, there exists a lot of "cross talk" between microbe and macrobe. So the concept of dysbiosis and probiosis (the holobiont made up of microbe and macrobe out of balance or maintaining balance) is relevant. In both cases, there is a delicate dance of signals and responses---in both directions---with an ancient history.

So it isn't simply that microbes see us as a surface; our epithelia tend to react to colonization. Perhaps this level of cross talk, with the evolutionary darwinnowing implied, is responsible for the small percentage of "microbial juvenile delinquents" (which get most of the press) observed.

As in one of Elio's earlier Talmudic Questions, a related question is why archaea have not been established (though there are some gray areas) as pathogens...because there are plenty of archaea in mutualistic associations with macrobes. It's not that (as one reader put it) "archaea don't eat meat." The mutualistic associations imply the same coevolutionary history under discussion. So why no frank pathogens?

Donne wrote that no man is an island. In a similar way, for good or for ill, we coexist with a microverse of prokaryotes...and are interacting incessantly. If only, as Joshua Lederberg famously wrote, we knew the frequency! Work continues, by wiser heads than mine.

Does God ever answer?

A perhaps related question is, why have so few pathogenic microbes learned to lie dormant until after their hosts have reproduced?

And, what really happens post-mating in senescent salmon and octopoda, pathogenically?

We simply don't know enough about commensals to say that they aren't pathogenic. Many microbes are opportunistic, sort of like friends who stab you in the back when they see the opportunity to dominate or survive at your expense. I believe that all life-froms are parasitic in nature and we just have to let time pass for discoveries on how they achieve their lifestyles. We shouldn't be depressed by this competitiveness though, as life is so fascinating to observe and we have such an enormous capacity to win in the battle against pathogens.

More would I think if it were not for the immune response of a potential host organism. This relates to the above question as I guess this requires a lot of energy/specialised traits. However, viruses - if anything - put the increase of energy use on to the host and not themselves. So this shouldn't be an issue for viruses.

When microbes evolve with their hosts for long periods of time, they learn to get along. In symbiotic relationships there is no point in the microbe causing damage to the host, so it is negatively selected for during evolution.


I think it's because successful pathogenicity requires so much energy and specialized traits that it is not "the best world" for a microrganism.

If it has the possibility to live in a "free" way, it will engage this form of life...

Anybody has any other idea?

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