Cyclic lipodepsipeptides display clear antagonistic activities against Gram-positive bacteria. Syringopeptin 22A, syringopeptin 25A, corpeptin-(SP), tolaasin, and WLIP-(V) all inhibit growth of Bacillus megaterium at similar concentrations (Emanuele et al., 1998; Grgurina et al., 2002; Bassarello et al., 2004; Rokni-Zadeh et al., 2012) while conflicting activities have been reported for syringomycin E and syringotoxin-(SM) (Lavermicocca et al., 1997; Scaloni et al., 2004). However, early reports of the biological activity of syringomycin Ps-CLPs were possibly performed with impure preparations that likely contained syringopeptins. Generally, syringopeptins have a higher activity than syringomycins and syringotoxins-(SM) against Rhodococcus and Micrococcus species (Lavermicocca et al., 1997).
The vast majority of Ps-CLPs is reported not to show antagonistic activities against Gram-negative bacteria (Lavermicocca et al., 1997; Grgurina et al., 2005; Lo Cantore et al., 2006; Andolfi et al., 2008; Sinnaeve et al., 2009a,b; Reybroeck et al., 2014). As a result, Pseudomonas CLPs tend to be considered ineffective against Gram-negative bacteria. This is generally attributed to the presence of the outer membrane or peptidoglycan layer which hinders access to the plasma membrane (Nybroe and Sørensen, 2004; Raaijmakers et al., 2006). However, things are not as clear-cut since CLPs of the tolaasin group (Lo Cantore et al., 2006) and the recently described xantholysin group (Li et al., 2013; Molina-Santiago et al., 2015) are able to inhibit Gram-negative bacteria. Furthermore, WLIP-(V) showed a modest activity against the Gram-negative Erwinia carotovora subsp. carotovora (Lo Cantore et al., 2006). To add to the apparent confusion, conflicting activities have been reported against some Xanthomonas species, whereby WLIP-(V) did (Rokni-Zadeh et al., 2012) or did not (Lo Cantore et al., 2006; Andolfi et al., 2008) show activity against this Gram-negative bacterium. This notwithstanding, it is clear that the activity of Ps-CLPs should not be considered as limited to Gram-positive bacteria.