Neutron reflection further substantiates that PlyC penetrates bilayers above a PS threshold concentration. for in the extracellular and intracellular milieu and as a scaffold for engineering other functionalities. DOI: http://dx.doi.org/10.7554/eLife.13152.001 is the bacterium that causes throat infections and other serious infections in humans. Antibiotics such as penicillin are used to treat active infections, but so-called strep throat infections often return after treatment. This is because can enter the cells that line the throat and hide from the antibiotics, which cannot enter the throat cells. Endolysins are enzymes produced by viruses that attack bacteria, and these enzymes target and destroy the bacterial cell wall. A previous study revealed that an endolysin known as PlyC could destroy bacteria on contact. PlyC and other endolysins have the potential to act as alternatives to common antibiotics, but before these enzymes can be developed as therapeutics, it is important to understand how they interact with human host cells. Like antibiotics, the PlyC endolysin was not expected to enter throat cells. However, Shen, Barros et al. have now discovered that not only can PlyC enter throat cells, it can essentially chase down and kill that are hiding inside. Other similar enzymes could Tucidinostat (Chidamide) not act in this way, and further studies confirmed that PlyC could move around inside a throat cell without causing it damage. Shen, Barros et al. also determined that PlyC has a pocket on its surface that binds with a specific component of the throat cell membrane, a molecule called phosphatidylserine. This interaction C which is a bit like a lock and key C grants PlyC access into the cell. While it is clear that PlyC eventually kills hiding inside throat cells, future experiments will aim to determine how PlyC moves around once inside an infected throat cell. Together, an understanding of how an endolysin enters cells and destroys hiding will contribute to the development of endolysins with broader activity, which can be used as Tucidinostat (Chidamide) alternatives to common antibiotics. DOI: http://dx.doi.org/10.7554/eLife.13152.002 Introduction is well known for its ability to proliferate within host cells (Barnett et al., 2013) and escape autophagic degradation (Sakurai et al., 2010). Notably, can be recovered from clinical specimens of excised human tonsils (Osterlund et al., 1997), even after antibiotic treatment. No effective approach has yet been identified that can specifically kill intracellular intracellular in biofilms was shown in vitro?(Shen et al., 2013), as was its therapeutic potential in an in vivo model of upper respiratory colonization (Nelson et al., 2001). Here, we investigate the ability of PlyC to target and control intracellular believed to be associated with streptococcal infections that are highly refractory to antibiotic treatment. Results PlyC possesses an inherent activity against intracellular colonization and invasion, we established a co-culture model of human epithelial cells and strain D471 to differentiate non-adherent, adherent, and intracellular streptococci. In experiments with human epithelial cell lines A549 (Figure 1figure supplement 1) or Detroit 562 (data not shown), rates of adherence ranged from 1?to?5% of the inoculum and rates of internalization ranged from 1?to?10% of the adherent streptococci, which are consistent with previous in vitro (PlyC (Nelson et al., 2006); B30 (Donovan et al., 2006); and Ply700 (Celia et al., 2008)) were evaluated for activity against intracellular CFUs. We then assessed PlyC in a co-culture with primary human tonsillar epithelial cells grown from UPA Tucidinostat (Chidamide) a tonsillectomy as a more clinically relevant model since these cells are known to be the major reservoir for recurrent pharyngotonsillitis (Osterlund et al., 1997). Roughly 90% of intracellular were eliminated when treated with 50?g/ml PlyC (Figure 1b), similar to the effect in Tucidinostat (Chidamide) immortalized A549 epithelial cells, although the lower dose treatments did not demonstrate significant killing. At present, Tucidinostat (Chidamide) it is not known if the differences in efficacy are due to differences in the?distribution of cellular receptors between the cell types or other phenotypic differences. Nonetheless, these data indicate that the native PlyC holoenzyme can be internalized by mammalian cells and that the endolysin retains bacteriolytic efficacy against in the intracellular environment. Open in a separate window Figure 1. PlyC eliminates intracellular in a dose-dependent manner.(a) were enumerated as colonies on agar plates that had been incubated with serial dilutions of cell lysates. (b) A primary tonsillar epithelial cell co-culture was treated with 10 g/ml penicillin and 200.