In the past few centuries, our understanding of bacteria has progressed from mysterious medieval vapours, to the microscopic "animalcules" of van Leeuwenhoek, to the germ theory of disease à la Pasteur, to the realizations that bacteria outnumber us within our own bodies and that good "probiotic" bacteria actually make us healthier.  Now, a new study seems to have discovered a Batman bacterium.  Well, technically, the bacterium was already well-known; the discovery was to show that this prokaryotic Bruce Wayne is, in fact, Batman.

What are the three defining characteristics of Batman?  First, you don't want to mess with Batman, for he will mess you up.  Second, Batman is a benevolent guardian.  Behave yourself, and all will be fine.  Third, Batman is multidimensional.  He is dark, his morality complex.  Given those defining characteristics, I submit to you, courtesy of Basler et al.1, that the bacterial species Pseudomonas aeruginosa is, indeed, Batman.
An electron micrograph of Pseudomonas aeruginosa in its natural environment.

The bacterial world is a tough one, pockmarked by chemical and biological warfare.  (Anthrax, after all, is a completely natural bacterium).  In this microscopic world, one of the most common weapons is the type VI secretion system (T6SS), which blasts through opponent bacterial cell walls and delivers (all-natural) biological weapon warheads.  P. aeruginosa, our Batman, has stockpiles of T6SS missiles, and it also has a missile defense system to deflect the T6SS of aggressors.  UN inspectors are still trying to determine how exactly this defense system works, but Basler et al. suggest it may involve super-strong cell wall armor, a kind of bullet-proof vest, in addition to antidotes for any poison that does get in. Mix P. aeruginosa with other T6SS-packing bacteria, such as Vibrio cholera or Acinetobacter baylyi, and P. aeruginosa wins, pilli down.  So Batman feature one – badassness: check.

However, if you mix P. aeruginosa with well-behaving bacteria that are not firing off T6SS's, it turns out that P. aeruginosa keeps its own T6SS holstered.  Even if you disarm the normally aggressive V. cholera by mutating its T6SS genes, P. aeurginosa now leaves it well-enough alone.  P. aeruginosa will mess you up, but only if you shoot first.  Contrast that with V. cholera, the Yosemite Sam of the bacterial world, which shoots off its T6SS at random but without P. aeruginosa's deadly precision.  Batman feature two – benevolence: check.
Vibrio cholera has a bit of trouble controlling its T6SS

As for the third feature of Batman – moral ambiguity – well, remember that P. aeruginosa is known to attack humans as well.  We should also note that P. aeruginosa can be rather high-strung; every once in a while one P. aeruginosa will flash its T6SS (presumably by accident, but we can't discount the possibility that it's showing off), which can spark violent duels with its neighboring sister cells.  Batman feature three: check.

So what exactly is going on here?  What role does this bacterial Batman play in microbial Gotham?  The study authors suggest that P. aeruginosa may play a kind of sheepdog role, protecting other bacteria within the same biofilm from aggressors.  In return, the other bacteria may help break down food that P. aeruginosa couldn't eat on its own.  However, microbial ecology is a very young field, and we can really only speculate what trauma P. aeruginosa went through as an evolutionary child to create this superhero.  At any rate, this discovery is yet more evidence of how complex the microscopic world really is.  On a more practical level, by understanding what makes P. aeruginosa unsheath its T6SS, we might be able to re-establish a truce when this bacterium does cause infections.  Eventually, we might even figure out how to hire it as a mercenary to supplement our own immune systems.  I hear Basler et al. are working on a bat signal.

1. Basler, M., Ho, B.T.&Mekalanos, J.J., 2013. Tit-for-Tat: Type VI Secretion System Counterattack during Bacterial Cell-Cell Interactions. Cell, 152(4), pp.884–894.