#legionella

There's been an outbreak in Sydney, NSW, Australia recently

ANd so triggers the review tonight.

There's 2 organisms to be aware of, one is the classic one associated with water reservoirs like air conditioners. this one is usually blamed for outbreaks. the other is less thought of, which is found in potting mix, so you might see this come up in avid gardeners (always wear masks)

Image Source; CDC

Legionella is a gram negative rod/bacilli, coming in a couple of different flavours or species. Whichever species is the causative agent, both can cause severe pneumonia, or Legionnaire's disease. So named as the "OG" known outbreak was at an American veterans' convention in the 1970s. Often seen with silver staining, so think legion think silver. Incubation time: 2-10 days.

They can also cause a milder version, which in the States is called Pontiac fever ("walking pneumonia"), which can feel flu like and rarely requires antibiotics, most won't necessarily present to hospital. So named as the index case was in Pontiac Michigan in the 60s. No particularly romantic story behind this. Incubation time of 1-3 days. Usually self resolving. Legionella pneumophila: found in water reservoirs like air conditioning cool towers etc. likes to contaminant stagnant water. Legionella longbeachae: what a great name. this one is found in potting mixes. So named as it was originally found in Long Beach California. Transmission is airborne --> inhalation of particles via airconditioners etc According to the CDC, <5% of those exposed to legionella while develop pathology (the rest will clear it).

Groups at increased risk of infection from Legionella: - pretty much the same as the at risk groups for severe COVID - immunocompromised or suppressed (on long term therapy/immunosuppression, diabetics, organ transplant recipients etc, HIV) - elderly (immune system not as optimal) - anyone with chronic lung disease like COPD or emphysema Investigation - as routine investigations for pneumonia, vital signs, examine findings, history (are you a gardener??), CXR, inflammatory markers, sputum culture and if you're a physician, you always end up requesting the urinary antigens which come back positive 50% of the time (even less if empirical antibiotics were started before these were sent off and they aren't positive for longbeachae - better to just give empirical therapy).

Treatment: Usual ABx for atypicals: doxycycline 100 mg BD for 7-10 days or azithromycin 500 mg for 3-7 daily. in severe Legionnaires' disease, extend azith to 7-10 days (no longer as it has a long intracellular half life).

Take homes:

Source: Australian Goverment

Resources: NSW Health

Usually free living amoebae PAC-man eat up bacteria om nom nom

But L. pneumophila was like “how about you eat us and then we multiply in your stomach and make you explode, thus freeing us into the water again😌”

Bad Neighbours

In bacterial communities, competition for resources can be fierce. Legionella pneumophila, the bacterium responsible for Legionnaire’s disease, a severe condition resembling pneumonia, shares contaminated plumbing systems with many potential rivals. While it produces no antibiotics or toxins to fight them off, it can still inhibit the growth of other Legionella species, using a molecule named HGA. On the plate shown here, HGA secreted from the line of L. pneumophila in the centre prevents the growth of other Legionella nearby. L. pneumophila is also sensitive to HGA, but a sophisticated mechanism protects it from self-harming: the bacteria become resistant to HGA when their population density is high, and only secrete it under those conditions, allowing them to safely eliminate competitors as needed. For its human hosts, removing Legionella from water supplies is an enduring challenge, but understanding how these bacteria interact with others could suggest new ways of tackling the problem.

Written by Emmanuelle Briolat

Image from work by Tera C Levin, Brian P Goldspiel and Harmit S Malik

Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, WA, USA

Image originally published under a Creative Commons Licence (BY 4.0)

Research published in eLife, May 2019