How do Different Metals Affect Legionella Growth?

Otto Schwake, PhD, Department of Civil and Environmental Engineering at Virginia Tech, lead researcher from the Flint Water Study team, explains how metals affect the growth of Legionella.

Interview Transcript (slightly modified for readability)

“One of our original ideas in the core of our hypothesis for a grant we put together and received, related to Legionella in Flint, was that corrosive water would interact with iron pipes [and] would leach iron into the water. Legionella have high requirements for iron [and] normally they do a good job of getting inside a host cell, like an amoeba, or a human macrophage. They can leach iron straight from the host, but without that, they have a high requirement for it that makes it a little tricky for them to, not survive, but, grow without the presence of high iron.

We have this idea that leaching of iron caused by the corrosive water in Flint would make it easier for Legionella to survive or grow. This is where it gets a little complicated; it’s not as straightforward as that. Basically, when we looked before, in [the] fall, we had high levels compared to 'normal' drinking water [levels] of iron throughout the city, [in] small buildings [and] large buildings, you know a little variable, but [there were] relatively high levels of iron, like we had predicted. But, when we tried to correlate concentrations of Legionella with concentrations of iron, we couldn’t establish a connection. So in some buildings, we actually saw that as iron went up, Legionella numbers actually went down, so counterintuitive to our prediction.

But we think that there might be some other factors in play. In water, you have a rich environment, a microbial community, and there’s a lot of microbes competing for iron and different microbes have different tolerances for iron, not just for use. Also at some point iron becomes toxic. We think there might be a nonlinear phasic relationship where when iron hits a threshold, Legionella might be able to use it or in the absence of a non-competing organism, iron might be able to have this impact on Legionella, this positive impact [that] we predicted. So, we’re really going to be focusing in on that and trying to look more at different physiochemical factors in addition to iron to see if we can solve the puzzle of why Legionella and iron didn’t quite mesh like we thought [they] would.

It's widely thought, it’s been reported in a number of studies, very anecdotal, [and] we talked to a lot of building architects or engineers [who said] 'use copper plumbing, it’s antibacterial.' It’s, again, like everything in science, like everything in microbiology, it’s more complicated than that. Eventually what happens to most piping, most plumbing, [is that] it develops a layer of scales, so what’s in the plumbing doesn’t get into the water. Even copper piping at some point will stop introducing much copper into the water.

Personally, I’ve ran multiple experiments looking at this, and under the right conditions, Legionella interacts with copper plumbing almost exactly like PVC, an inert plumbing that’s thought to be conducive for Legionella. Still, many studies have shown that at least in new buildings or in certain conditions, copper plumbing can release copper particles into water that have some sort of impact on Legionella, [making] it difficult for them to survive in the water that’s going through the plumbing, but more importantly, to grow in biofilms growing on the copper. So again, that’s one of the metals [that] we were really focusing in on as we look for it in Flint.

Between fall 2015 and March this year, we saw a significant, not sure if its biologically relevant, but a significant and a definite increase in copper levels [in water]. So [that may be] another potential indicator of how the chemistry might be improving in the water in Flint.”