People with schizophrenia harbour different collections of oral bacteria than those without the mental illness, according to two new studies. The findings suggest that changes in mouth and throat germs may be associated with the disease and could offer a new tool for diagnosis involving a test as simple as a saliva sample.

Lactic acid bacteria, or Lactobacillus gasseri, was found to be at least 400 times more abundant in schizophrenic patients than control subjects, according to a study published last week in the journal PeerJ.

“Essentially we wanted to know whether the microbiome (the totality of microbes in the human body) was significantly different in individuals with schizophrenia versus controls,” said lead author Eduardo Castro-Nallar in an email, “because there’s evidence that the microbiome may play a role in other behavioural conditions such as depression and anxiety.”

Lactobacillus gasseri is naturally found in human breast milk and yogurt and is a probiotic often sold as a health supplement or weight loss pill, but the researchers did not link breastfeeding, yogurt or probiotic supplements to schizophrenia.

In research published in the Schizophrenia Bulletin, scientists at Johns Hopkins School of Medicine and the Stanley Research Program at Sheppard Pratt Health System, also found a viral form of Lactobacillus, the phiadh phage, which infects bacteria and alters their replication, was significantly higher in individuals with schizophrenia.

“The level of Lactobacillus phage phiadh correlated with the prevalence of immunological disorders . . . which has been shown in animal models to alter the microbiome,” the Johns Hopkins study said.

As a probiotic, Lactobacillus is considered beneficial to the human body, enhancing the immune system and aiding digestion. Found chiefly in the intestines, it has also been linked to the regulation of emotional behaviour and anxiety.

Castro-Nallar, of George Washington University’s Computational Biology Institute, noted that a lactate-utilizing bacterium present in human feces and an opportunistic fungus were also present at higher levels in the schizophrenic group. His research looked at only a small sample of subjects, 16 with schizophrenia and 16 without. The Johns Hopkins study relied on 41 schizophrenic subjects and 33 controls.

There are about 100 trillion bacteria in an average person’s body and these bacteria are affected by everything from food to cigarette smoking to pollution.

Both journal articles said further study was needed, particularly to determine whether changes in bacteria in the human body contribute to schizophrenia, are a result of schizophrenia or have no connection at all.