How dried-out bacteria revives with a vengeance in hospitals once water is added

An increasingly antibiotic-resistant pathogen that can survive months dehydrated on hospital surfaces may hold the key to new anti-bacterial treatments and preserving probiotics in pills.

A new study has revealed how the bacteria Acinetobacter baumannii survives without water on surfaces for months, an ability which has helped it become a leading cause of hospital-acquired infections.

Scientists now know it survives in this dried-out state by producing “hydrophilin” proteins that protect it against water deprivation, and it causes more virulent infections after being dried and rehydrated.

The discovery, reported in the journal Cell Host & Microbe, could guide new strategies to eliminate Acinetobacter from surfaces by targeting these hydrophilin proteins, and could even translate into a fresh strategy for preserving dried probiotics.

This is excellent news because Acinetobacter has been recognised by the World Health Organization (WHO) as one of the highest-priority bacteria for which new antibiotics are needed, due to its growing antibiotic resistance.

“The fact that A. baumannii contaminates hospital surfaces and is extremely difficult to get rid of puts it into close contact with very vulnerable patients,” says senior author Eric Skaar, professor of Pathology, Microbiology and Immunology at Vanderbilt University, in the US.

The findings might also have applications for preserving protein and live bacteria-based pharmaceuticals (probiotics) that are dried and packaged into pill form – as only a fraction normally survive.

“A challenge for probiotics is getting enough bacteria through the stomach and into the gut,” explains Skaar. “If you put these proteins from Acinetobacter into a probiotic, that organism would be much more likely to survive the desiccation (drying) process and come out of the pill alive.”

The team of microbiologists and chemists has found that Acinetobacter baumannii could survive more than seven months of desiccation and that when it was dried and then rehydrated, it caused more virulent infections in mice.

They also found that recently isolated clinical strains of Acinetobacter were 10-times more resistant to desiccation compared with an older laboratory strain.

Using genetic screening, they then discovered two “desiccation tolerance proteins” – which they named DtpA and DtpB – that have an unusual amino acid sequence of repeating units.

“The protein sequence really surprised us, and we figured out pretty quickly that DtpA and DtpB share these unusual features with a group of proteins called ‘intrinsically disordered proteins’ that are present in organisms known to be extraordinarily resistant to water starvation,” says Skaar.

The list of organisms includes tardigrades, nematodes, yeast and plant seeds, but DtpA and DtpB are some of the first intrinsically disordered proteins to be characterised in bacteria.

Unlike typical proteins, which fold into a three-dimensional structure that dictates their function, these intrinsically disordered proteins do not exist in a fixed conformation in solution.

“It was pretty cool to figure out that Acinetobacter is using the same strategy to resist water deprivation as tardigrades, which are among the most resilient animals known and have even survived exposure to outer space,” says Skaar.

The team then demonstrated that this desiccation protection could be extended to a different, probiotic strain of bacteria by expressing DtpA inside the bacterial cells.

The researchers found that by drying, or heat-inactivating, a purified protein enzyme in the presence of DtpA, then the enzyme’s activity could also be maintained when it wouldn’t otherwise.

“We think these proteins may have a valuable commercial application for preserving activity of protein and probiotic therapeutics,” says Skaar.