New Bacterial Weaponry- the toxic balloons

The Role of Vesicles in Microorganisms

The scientific community was originally dismissive of the first discoveries (over a half-century ago) that several different forms of cells and species, including microbes, could release vesicles into the atmosphere. In recent years, we've discovered that vesicle release is an active mechanism and that these vesicles hold essential information, can fuse, and deliver contents to other cells. Various experiments are only now revealing the possible implications of this operation. This new research discovers that 'toxic bubbles' formed by a bacterium found in our bodies can be related to premature labor and stillbirth. Essentially, it is the dynamic surface features of the micro-organisms that specifically determine the latter's interaction with their environment throughout the cell growth and division. Vivid surface modifications are acquired for both adjacent and remote- controlled functions such as nutrients uptake, defending the competing microbes, or offering resistance in response to a host immune system; therefore, facilitating the micro-organisms to favorably adapt to their unique surroundings (Mashburn-Warren & Whiteley, 2006). It is noteworthy that the extrusion of these bioactive Membrane Vesicles (MVs) from the cell surface into extracellular milieu is evolutionary conserved and is a universal process that occurs across all microbial taxa (bacteria, archaea, fungi, and parasites) (Deatherage & Cookson, 2012). Outer MVs (OMVs) are released both in vitro as well as in vivo during infection, adding a shocking discovery to the putative role of these surface organelles in the studies related to the microbial physiology and pathogenesis. Now it is evident that these vesicles are the carrier and courier of communication signals, enzymes, toxins, and antigens recognized by the innate and adaptive immune systems.

Mechanisms of Vesicles Biogenesis

Vesicle biogenesis is a ubiquitous process and is extensively researched in the case of Gram-negative bacteria whereas have been completely overlooked in Gram-positive bacteria because both the bacterial genre are structurally different. MVs bud off from the outer membrane of Gram-negative bacteria, which is not observed in their Gram-positive counterparts (Beveridge, 1999). Moreover, archaea, fungi, and other eukaryotic cells also release MVs; suggesting that the shedding of OMVs is an evolutionarily conserved secretory pathway of proteins and pointing the possibility of its occurrence in the case of Gram-positive bacteria. Therefore, mechanisms for outer membrane vesiculation can be evaluated by comparing similar budding mechanisms across diverse biological domains.

The Assessment of OMVs Biogenesis

The assessment provides new insight into OMVs biogenesis that it encompasses breaking the interconnection between the outer membrane and the inner peptidoglycan wall, modifying membrane by inducing localized membrane curvature, accumulating/excluding specific proteins, and releasing bioactive vesicles. Two general mechanisms have been proposed regarding the release of MVs in Gram-negative bacteria, According to the first proposition, OMVs are liberated due to the rapid expansion of both the membranes, outer grows more rapidly than the underlying peptidoglycan layer (Bernadec, 1998), On the other hand, it was proposed that MVs buds off when murine hydrolases subsequently accumulate in the periplasmic space thereby increasing the turgor pressure and degrade peptidoglycan fragments (Lommatzsch et al., 1997). Vesicle formation may further assist by penicillin-binding proteins and lipoteichoic acid synthase which contribute to cell-wall biogenesis.

Functional Aspects of OMVs

Bacterial species constantly secrete a spherical, bilayered proteolipids, referred as OMVs which is about on an average 20–200 nm in diameter. Their structural compositions are lipopolysaccharide, lipids, soluble or membrane-associated proteins, genetic materials, and other factors associated with virulence (Lee et al., 2008). Although functions of isolated membrane-derived vesicles in Gram-positive bacteria were not apparent in early studies (Dorward & Garon, 1990), recent studies have reported the associated functions with the released membrane-derived vesicles by Gram-positive Staphylococcus aureus, Mycobacterium ulcerans, and Bacillus spp. (Dorward & Garon, 1990; Marsollier et al., 2007; Lee et al., 2009; Kim et al., 2009), further supporting the fact that vesicle production is a universal phenomenon among microbial life. Lee et al. adopted a proteomic-based approach to characterize the MVs produced naturally pathogenic S. aureus during the bacterial growth revealing that several common features of OMVs are shared by Gram-positive and Gram-negative bacterial strains (2009). They characterized extensively S. aureus-derived MVs components which includes 90 vesicular proteins. Further analysis of identified proteins demonstrated that the virulence of S. aureus-derived MVs is due to present (in excess) of specific extracellular or surface-associated proteins involved critically in various pathologies for systemic infections in the human body. The identified virulent factors were toxins, adhesins, proteolysin, coagulase, and other related enzymes.

OMVs and Pregnancy Complications

A remarkable revelation made by Modi and colleagues that bacterial invaders compromise pregnancy in an animal model (Surve et al., 2016). They elicited the pathogenic mechanisms of Group B Streptococcus- derived MVs that bear a complement of pore-making toxins and virulence associated enzymes. These vesicle components disrupt the mechanical integrity of the fetal membranes causing chorioamnionitis, pre-term birth or still births in pregnant mice, which further signifies high probability of the same outcome in humans. The issue gained importance because around 30 percent of pregnant women worldwide have benign Group B Streptococcus (Strep B) inhabiting the lower genital tract, often without causing any problem. However, the above study provides evidence that the bacteria in pregnant mice produce protein-filled balloons or OMVs that have a remote-controlled function. These balloons migrate up into the uterus causing extensive inflammation and disrupt the connective tissue of the fetal membrane reducing its mechanical strength which may cause premature rupture of the amniotic sac. If the damage is more, stillbirth may result. The proteomic analysis of these toxic balloons indicated a presence of corrosive proteins that trigger cell death. Although the underlying mechanisms of OMVs biogenesis are unclear, they may be used in turf wars against other bacteria.

Implications for Research and Prevention

Researchers are now looking this discovery as an option to develop a vaccine against Strep B. According to a report published in Science News, medical professionals are testing 35-37 week pregnant women for it (Mcdermott, 2016). If test results are positive, antibiotics are recommended to Strep-positive patients during labor to eliminate any risk of infection from spreading to newborns. However, practitioners are now considering an earlier diagnostic test in order to ultimately prevent possible stillbirths. Needless to say that this expanding area of research calls for more focus on the reliance of the reproductive tract in males and females on the microbiota that both facilitate and compromise fertility.

References

Bernadac, Alain, Gavioli, Marthe, Lazzaroni, Jean-Claude, Raina, Satish, & Lloubès, Roland. (1998). Escherichia coli tol-pal mutants form OMVs. Journal of bacteriology, 180(18), 4872-4878.

Beveridge, Terry J. (1999). Structures of gram-negative cell walls and their derived MVs. Journal of bacteriology, 181(16), 4725-4733.

Deatherage, Brooke L., & Cookson, Brad T. (2012). Membrane vesicle release in bacteria, eukaryotes, and archaea: a conserved yet underappreciated aspect of microbial life. Infection and immunity, 80(6), 1948-1957.

Dorward, David W., & Garon, Claude F. (1990). DNA is packaged within membrane-derived vesicles of gram-negative but not gram-positive bacteria. Applied and environmental microbiology, 56(6), 1960-1962.

Kim, Sang-Hyun., Kim, Keun-Su, Lee, Sang-Rae., Kim, Ekyune., Kim, Myeong-Su., Lee, Eun-Young, Gho, Yong Song, Kim, Jung-Woo, Bishop, Russel E., & Chang, Kyu-Tae. (2009). Structural modifications of OMVs to refine them as vaccine delivery vehicles. Biochimica et Biophysica Acta (BBA)-Biomembranes, 1788(10), 2150-2159.

Lee, Eun-Young, Choi, Dong-Sic, Kim, Kwang-Pyo, & Gho, Yong-Song. (2008). Proteomics in gram‐negative bacterial OMVs. Mass spectrometry reviews, 27(6), 535-555.

Lee, Eun-Young, Choi, Do-Young, Kim, Dae-Kyum, Kim, Jung-Wook, Park, Jung Ok, Kim, Sungjee, Kim Sang-Hyun, & Gho, Y. S. (2009). Gram‐positive bacteria produce MVs: Proteomics‐based characterization of Staphylococcus aureus‐derived MVs. Proteomics, 9(24), 5425-5436.

Lommatzsch, Jurgen, Templin, Markus F., Kraft, Angelika R., Vollmer, Waldemar, & Höltje, J. V. (1997). Outer membrane localization of murein hydrolases: MltA, a third lipoprotein lytic transglycosylase in Escherichia coli. Journal of bacteriology, 179(17), 5465-5470.

Marsollier, Laurent, Brodin, Priscille, Jackson, Mary, Korduláková, Jana, Tafelmeyer, Petra, Carbonnelle, Etienne, Aubrey, Jacques, & Leroy, C. (2007). Impact of Mycobacterium ulcerans biofilm on transmissibility to ecological niches and Buruli ulcer pathogenesis. PLoS Pathog, 3(5), e62.

Mashburn‐Warren, Lauren M., & Whiteley, Marvey. (2006). Special delivery: vesicle trafficking in prokaryotes. Molecular microbiology, 61(4), 839-846.

Mcdermott, Amy. (2016, September 1). Bacterial weaponry that causes stillbirth revealed. https://www.sciencenews.org/article/bacterial-weaponry-causes-stillbirth-revealed

Surve, Manalee Vishnu, Anil, Anjali, Kamath, Kshama Ganesh, Bhutda, Smita, Sthanam, Lakshmi Kavitha, Pradhan, Arpan, Srivastava, Rohit, Basu, Bhakti, Dutta, Suryendu, Sen, Shamik and Modi, Deepak (2016). MVs of group b streptococcus disrupt feto-maternal barrier leading to preterm birth. PLoS Pathog, 12(9), e1005816.