Page 14 - Volume 5, Issue 2, July 2012

© Benaki Phytopathological Institute
Skandalis et al. 42
clude regions of high polymorphism. Based
on pilot studies from the authors of this
manuscript (Skandalis et al. , 2010), we sug-
gest that microarray target probe design-
ing on hrc genes could be a valuable tool
for detection of a wide range of Gram-neg-
ative plant pathogens as well as for simul-
taneous infections. In support, Garrido et al.
(2006) already developed a diagnostic oli-
gonucleotide array for the detection of shi-
ga toxin-producing and enteropathogenic
E. coli strains based on the genetic variabil-
ity present at the enterocyte effacement
(LEE) pathogenicity locus, which contains
genes encoding for the T3SS and T3SS effec-
tor proteins.
Are secretion systems the key to a new
generation of antibiotics?
Development of resistance to antibiot-
ics that target bacterial viability, urged for
new antimicrobial development strategies
that target various pathways related to vir-
ulence, including toxin function, toxin deliv-
ery, regulation of virulence expression and
bacterial adhesion (reviewed in Clatworthy
et al. , 2007). Inhibition of these virulence fac-
tors would render hazardous bacteria non-
infectious only within the host, thereby re-
ducing the selective pressure for resistance
development, and would not perturb the
nontarget microflora. In this regard, T3SS
apparatus components that are linked to
the outer membrane or exposed, offer an
attractive substrate for inhibitor (antibiotic)
development. Hence, screening for T3SS in-
hibitors has involved four distinct approach-
es, yielding inhibitors of T3SS transcription,
assembly and specific effector protein se-
cretion (reviewed in Lee and Kessler, 2009).
In particular, small molecules termed In-
nate Parmaceuticals AB (INPs), and in partic-
ular acylated hydrazones of different salicy-
laldehydes were found to prevent molecule
translocation for Yersinia pseudotuberculo-
sis (Nordfelth et al., 2005), inhibit intracel-
lular growth but not invasion of Chlamydia
trachomatis into host cells (Muschiol et al.,
2009), inhibit the T3SS and virulence of Sal-
monella enterica serovar Typhimurium (Hud-
son et al., 2007) and finally inhibit secretion
assembly of Shigella flexneri T3SS. Effector
protein inhibitors were identified either –in
the case of pseudolipasin A - on the basis of
inhibition of host cell lysis by the P. aerugino-
sa ExoU cytotoxin effector or alternatively,
by screening for compounds, such as exosin
and 0433YC1-2, that permitted the growth
of yeast with induced expression of cytotox-
ic effectors of P. aeruginosa (ExoS) and Chla-
mydia pleumoniae (CopN). Determination of
3-D structures of T3SS components and ef-
fectors provide unexplored options for de-
signing antivirulence chemicals, including
next generation virulence inhibitors bind-
ing to adjacent, functionally relevant bind-
ing pockets of T3SS components to reduce
the chances of resistance development.
Conclusions
Historically, the genes coding for the T3SS
were identified in phenotypic screens of
mutants altered in their interaction with
other organisms (higher eukaryotes). The
identification of effectors in plant patho-
gens also had a similar starting point (trans-
fer of whole genome libraries from aviru-
lent to virulent strains and carrying out HR
screens on R gene differentials; Staskawicz
et al., 1984; Table 1). The discovery of the
T6SS followed the opposite route: its exis-
tence emerged from bioinformatic sourcing
of genomic data. It is conceivable that fu-
ture sourcing of genomic, proteomic, tran-
scriptome and structural database data may
point to new potential candidates, partic-
ularly for effectors. Possibly, new secretion
systems and new effectors may even be pre-
dicted, “the way Mendeleev had anticipated
characteristics of yet unknown elements”,
as is already the case with TAL effector DNA
binding specificities.
Large scale analysis of interactions of
effectors with different hosts by means of
Agrobacterium transient expression (Wro-
blewski et al., 2009; Lindeberg et al., 2012)
confirmed that their suppression and avir-
ulence function varied significantly, espe-