Salmonella exposed to space travel come back nastier.
  National Geographic:

When the bacteria—which had been safely isolated from the space crew—returned to Earth, scientists injected them into mice.

They found the space-faring bacteria caused death quicker and more often than Earth-restricted organisms.

The findings are concerning for future astronauts who will embark on
longer space missions farther away from Earth-based medical help,
experts say.

Genetic Transformations

Cheryl Nickerson is an associate professor of microbiology at
Arizona State University’s Biodesign Institute and lead author of the
study….

…When the bacteria returned to Earth, genetic sequencing showed that 167 genes and 73 proteins had been altered.

One protein, called Hfq, helped control more than a third of the
altered genes. Hfq regulates RNA—the code of bacterial life—during
stressful events. When activated, the protein previously had been shown
to strengthen several types of pathogens.

An technique called scanning electron microscopy also showed some Salmonella were starting to form biofilms, a protective slime layer.

On Earth, biofilms can grow on ship hulls and clog pipes, costing
industry billions of dollars. Biofilms also worsen some diseases and
reduce the effectiveness of many antibiotics.

Could it be….the cosmic rays?!