Tag: salmonella

Categories : AntibioticsTags :

Why Antibiotics can Fail Even against Non-resistant Bacteria

On By ModernMedia
Drug-resistant Salmonella. Credit: CDC

Antibiotics are indispensable for treating bacterial infections. But why are they sometimes ineffective, even when the bacteria are not resistant? In their latest study published in the journal Nature, researchers from the University of Basel challenge the conventional view that a small subset of particularly resilient bacteria are responsible for the failure of antibiotic therapies.

In certain infectious diseases caused by bacteria, antibiotics are less effective than expected. One example is infections caused by Salmonella bacteria, which can lead to illnesses such as typhoid fever. For many years, researchers believed that a small subset of dormant bacteria are the main problem in fighting infections. These so-called persisters can survive antibiotic treatment and cause relapses later. Researchers worldwide have been working on new therapies aimed at targeting and eliminating these “sleeping” bacteria.

In a new study, Professor Dirk Bumann’s team from the Biozentrum of the University of Basel challenges the prevailing concept that persisters are the cause of antibiotic ineffectiveness. “Contrary to widespread belief, antibiotic failure is not caused by a small subset of persisters. In fact, the majority of Salmonella in infected tissues are difficult to kill,” explains Bumann. “We have been able to demonstrate that standard laboratory tests of antimicrobial clearance produce misleading results, giving a false impression of a small group of particularly resilient persisters.”

Nutrient starvation increases Salmonella resilience

The researchers investigated antimicrobial clearance in both Salmonella-infected mice and tissue-mimicking laboratory models. The body’s defense mechanisms against bacteria often include reducing the availability of nutrients. The researchers have now revealed that in fact, this nutrient starvation is the main reason for Salmonella bacteria surviving treatments with antibiotics. The researchers assume that the same applies to other bacterial pathogens.

“Under nutrient-scarce conditions, bacteria grow very slowly,” says Bumann. “This may seem good at first, but is actually a problem because most antibiotics only gradually kill slowly growing bacteria.” As a result, the drugs are much less effective, and relapses can occur even after prolonged therapy.

Real-time analyses reveal misconception

The scientists used an innovative method to monitor antibiotic action in single bacteria in real time. “We demonstrated that nearly the entire Salmonella population survives antibiotic treatment for extended periods, not just a small subset of hyper-resilient persisters,” says first author Dr Joseph Fanous.

A major issue with the standard methods used worldwide for decades is their indirect and delayed measurement of bacterial survival, leading to distorted results. “Traditional tests underestimate the number of surviving bacteria,” explains Fanous. “And they falsely suggest the presence of hyper-resilient subsets of persisters that do not actually exist.” This misinterpretation has influenced research for many years.

Novel tools for antibiotics research

These findings could fundamentally change antibiotics research. “Our work underlines the importance of studying bacterial behaviour and antibiotic effects live and under physiologically relevant conditions,” emphasises Bumann. “In a few years, modern methods like real-time single-cell analysis will hopefully become standard.” Shifting the focus from persisters to the impact of nutrient starvation is an important step toward more effective therapies against difficult-to-treat infections.

The project is part of the National Center of Competence in Research (NCCR) “AntiResist”. The research consortium aims to develop innovative strategies to combat bacterial infections. Dirk Bumann is one of the directors of the NCCR “AntiResist”.

Source: University of Basel

Categories : Immune SystemTags :

How Macrophages Control an Uncooperative Meal

On By ModernMedia
A macrophage digesting a yeast cell (yellow). Credit: NIH

Certain pathogens such as Salmonella have developed strategies to protect themselves from the macrophages’ digesting attempts, causing severe Typhoid infections and inflammations. Scientists report in Nature Metabolism how the inter-organellar crosstalk between phago-lysosomes and mitochondria restricts the growth of such bacteria inside macrophages.

Signals from the digestion cell organelle

As scavenger cells, macrophages have a very prominent digestion organelle, the phago-lysosome, where engulfed microorganisms are commonly degraded into pieces and become inactivated. “It has long been known that the molecule TFEB (Transcription factor EB) is important for the regulation of the phago-lysosomal system. More recent evidence also suggested that TFEB supports the defense against bacteria,” said Max Planck Institute group leader Angelika Rambold.

She and her team wanted to understand how exactly TFEB mediates its anti-bacterial role in macrophages. They confirmed earlier findings showing that a broad range of microbes, bacterial and inflammatory stimuli activate TFEB and thus the phago-lysosomal system.

“It made sense that pathogen signals trigger TFEB as macrophages need a more active digestion system quickly after they devour a meal of bacteria. But, interestingly, the experiments also revealed an additional strong effect of TFEB activation on another intracellular organelle system — mitochondria. This was completely unexpected and novel to us,” said Angelika Rambold.

Instructing mitochondria to increase anti-microbial activity

Composed of inner and outer membranes, mitochondria are the primary sites of cellular respiration and release energy from nutrients. Moreover, the mitochondria in immune cells were recently identified as sources of anti-microbial metabolites.

By using a broad experimental tool set, the investigators identified the pathway controlling an unexpected crosstalk between lysosomes and mitochondria. “Macrophages make use of extensive inter-organellar communication: the lysosome activates TFEB, which shuttles into the nucleus where it controls the transcription of a protein called IRG1. This protein is imported into mitochondria, where it acts as a major enzyme to produce the anti-microbial metabolite itaconate,” explained Angelika Rambold.

Exploiting organelle communication to control bacterial infections

The researchers investigated whether they could make use of this newly identified pathway to control bacterial growth. “We speculated that activating this pathway could be used to target certain bacterial species, such as Salmonella,” said Angelika Rambold. “Salmonella can escape the degradation by the phago-lysosomal system. They manage to grow inside macrophages, which can lead to the spreading of these bacteria to several organs in an infected body,” explained Alexander Westermann, collaborating scientist from the University of Würzburg.

When the researchers activated TFEB in infected macrophages in mice, the TFEB-Irg1-itaconate pathway inhibited the growth of Salmonella inside the cells. These data show that the lysosome-to-mitochondria interplay represents an antibacterial defense mechanism to protect the macrophage from being exploited as a bacterial growth niche.

In light of the increasing emergence of multi-drug resistant bacteria, with more than 10 million expected deaths per year by 2050 according to the various expert groups, it becomes important to identify new strategies to control bacterial infections that escape immune mechanisms. A promising path could be to use the TFEB-Irg1-itaconate pathway or itaconate itself to treat infections caused by itaconate-sensitive bacteria. According to the researchers from more work is needed to assess whether these new intervention points can be successfully applied to humans.

Source: Max Planck Institute of Immunobiology and Epigenetics