In order for the pathogen, a mere tiny bacterium, to gain leverage during infection against highly prepared immune cells, they must evolve to be stealth and well-equipped for battle. Specifically, intracellular pathogens, which replicate within host cells, have evolved survival strategies to evade host antimicrobial mechanisms by modulating, or taking control, of these mechanisms.
Some intracellular bacteria, like Brucella abortus, replicate within host membrane-bound vacuoles, or enclosed intracellular compartments. After host cells engulf bacteria from the extracellular milieu, the bacteria within the bacteria-containing vacuole (BCV) are recognized as foreign invaders and can be redirected for degradation by autophagy.
Autophagy has earned its right to be a current buzzword due to its inherent badassedness. Autophagy is an intracellular process of capture and destroy (by lysosomal degradation, explained below) of damaged organelles, protein aggregates, and cytosolic content. However, in the last few years autophagy has been identified to be an important player in innate immunity, or capture-and-destroy-pathogen-immunity.
When the bacteria within the BCV are recognized as foreign, the vacuole is engaged by autophagy and the trafficking of this intracellular compartment is directed towards the lysosome (degradation chamber, death compartment, end-of-the-road). The lysosomal compartment is the point of no return for most bacterium; here is where bacteria are faced with lowered-pH, antimicrobial peptides, and reactive oxygen and nitrogen species. Thus, the host cells use autophagy to control intracellular survival and replication of pathogens.
How can the bacteria fight for their right to replicate?
Multiple intracellular bacteria have evolved mechanisms to redirect or manipulate the autophagic pathway, including; Listeria monocytogenes, Shigella flexneri, Coxiella burnetii, Yersinia pestis, pseudotuberculosis, Porphyromonas gingivalis and our star player today, Brucella abortus.
Brucella abortus is the causative agent of brucellosis, a worldwide zoonosis (transmitted from animal to humans) with a significant health and economic burden. Brucella is transmitted by ingesting infected food, direct contact with an infected animal or inhalation of aerosols.
To cause disease, Brucella enters the host cells and replicates within the endoplasmic reticulum (ER), remember that organelle? It has the ability to not only replicate in a variety of host cells, but can replicate in macrophages and dendritic cells, both immune cells that are programmed to kill foreign invaders. In order to successfully replicate within the ER, Brucella has evolved mechanisms to manipulate intracellular trafficking processes. First, Brucella resides in the Brucella-containing vacuole (BCV) that traffics, with very minimal fusion with the lysosome, to the ER. Next, Brucella replicates within the modified ER-derived BCVs – and this is where current knowledge of the Brucella life-cycle ends.
However, Tregei Starr, et al. studied the later stages in infection and reports the modified ER-derived BCVs are converted into BCVs with autophagic features, including multiple autophagy-related protein markers, Beclin1-, ULK-1, and ATG14L. Strikingly, they have shown the BCV conversion to autophagosomal compartments (pictured below) completes the intracellular cycle of Brucella and promotes cell-to-cell spread.
Representative TEM images of macrophages infected with B. abortus 2308 for 72 hr. Arrows indicate double-membrane structures on BCVs - autophagosomes. Scale bars represent 500 and 200 nm.
Interestingly, these autophagic compartments were ATG5, ATG4B, ATG16L1, ATG7, and LC3B-independent – which are all markers of autophagosomes. However, there has been evidence of an ATG5/ATG7-independent alternative autophagy-like process that is dependent on Beclin1 and ULK1, like the Brucella-containing autophagosomes. Therefore they hypothesize that Brucella engage autophagosome-like rearrangements dependent upon a subset of autophagy-associated proteins, that may contribute to an unconventional autophagic response… that has yet to be identified deep breath.
Needless to say, these autophagosome-like compartments seem to be contributing to Brucella release and spread – further contributing to the paradigm of pathogens hijacking intracellular pathways for subversion of degradation - and my tiny scientist heart is happy knowing the complete Brucella intracellular life cycle.
Starr T, Child R, Wehrly TD, Hansen B, Hwang S, López-Otin C, Virgin HW, & Celli J (2012). Selective subversion of autophagy complexes facilitates completion of the Brucella intracellular cycle. Cell host & microbe, 11 (1), 33-45 PMID: 22264511