Genomewide O e exp ession Sc een o Fos omycin Resis ance in
Esche ichia coli: Mu A Con e s Clinical Resis ance a Low Fi ness Cos
Alejand o Couce,
a
Alejand a B iales,
b
Alexand o Rod íguez-Rojas,
a
Coloma Cos as,
a
Ál a o Pascual,
b
and Jesús Blázquez
a
Cen o Nacional de Bio ecnología, Consejo Supe io de In es igaciones Cien í icas, Mad id, Spain,
a
and Depa amen o de Mic obiología, Uni e sidad de Se illa, Se ille,
Spain
b
To de e mine whe he he o e exp ession o ch omosomal genes can con e os omycin esis ance, genomewide sc eening o a
comple e se o 5,272 plasmid-exp essed open eading ames o Esche ichia coli (ASKA collec ion) was pe o med. Majo esul s
a e ha (i) no clinical le el o esis ance is achie ed by o e exp essing ch omosomal genes, excep mu A; (ii) his le el is eached
a a low i ness cos ; and (iii) his cos is much lowe han ha imposed by o he mu a ions con e ing os omycin esis ance.
The eme gence o an ibio ic esis ance mu an s in a bac e ial
popula ion is shaped by se e al ac o s, and bo h he mu a ion
a e and he i ness cos o esis ance a e pa icula ly ele an (2).
I esis ance came a a high i ness cos , he g ow h a e would no
be enough o o se he clea ance imposed by he hos o o p e en
he bac e ia om being ou compe ed by i e suscep ible bac e ia
once he an ibio ic is emo ed (2).
Fos omycin (Fos) is a b oad-spec um bac e icidal an ibio ic
ac i e agains bo h G am-posi i e and G am-nega i e bac e ia
(7). Con enien ly, Fos ea men s ha e shown a ela i ely low
likelihood ha esis an mu an s will pe sis in i o, p obably due
o he high biological cos o esis ance mu a ions, leading o good
he apeu ic e ec i eness (19).
Fos esis ance is acqui ed mainly by educing he cell’s d ug
up ake (4, 10, 11, 22), al hough ac i e e lux, a ge al e a ions,
and plasmid-encoded esis ance ha e also been desc ibed in Esch-
e ichia coli and o he species (18, 21, 23, 24). Resis an mu a ions
usually en ail a mode a e o high i ness cos (1, 15, 19), as well as
educed i ulence (8, 13, 15), which has been in oked o explain
he low p e alence o esis an s ains (19). Despi e his, a signi i-
can inc ease in esis ance has been ecen ly desc ibed a e an i-
bio ic p essu e in he communi y (20), sugges ing ha he e may
be o he , uniden i ied, ways o a ain less cos ly high-le el Fos
esis ance.
The genome o E.coli ha bo s a subs an ial ese oi o esis-
ance genes whose o e exp ession can dec ease suscep ibili y (22).
The comple e E.coli open eading ame (ORF) ASKA lib a y has
al eady been sc eened o esis ance o 237 oxins and an ibio ics.
Howe e , Fos was no included among he d ugs used o sc een
o esis ance. Gi en he enewed in e es in Fos ea men , we
explo ed he capaci y o he o e exp ession o ch omosomal E.
coli genes o con e clinical le els o Fos esis ance by sc eening he
comple e ASKA lib a y (12).
Genomewide o e exp ession sc eening o Fos esis ance.
The comple e ASKA lib a y was eplica ed in duplica e in 96-well
pla es con aining Lu ia-Be ani (LB) b o h plus chlo amphenicol
(50
␮
g/ml) wi h and wi hou isop opyl-
␤
-D- hiogalac opy ano-
side (IPTG) o a inal concen a ion o 100
␮
M and incuba ed
o e nigh a 37°C. The ansc ip ion o he cloned genes o he
ASKA collec ion is unde he con ol o he P ac p omo e , which
is induced by he addi ion o IPTG (12). A 5-
␮
l sample om each
well was spo ed on o an LB aga pla e con aining ei he 32
␮
g/ml
Fos ( he EUCAST b eakpoin o En e obac e iaceae [h p://www
.eucas .o g/clinical_b eakpoin s/]) o no an ibio ic. Pla es we e
incuba ed o 24 h a 37°C. Only 1 o he 5,272 clones was able o
g ow on Fos. The clone, con aining plasmid pCA24N-mu A, was
isola ed om he o iginal ozen 96-well pla e and in oduced by
ans o ma ion in o s ain MG1655. Th ee independen ans o -
man s we e isola ed, and Fos esis ance was e i ied as indica ed
abo e. The p esence o he wild- ype mu A gene in he plasmid
was e i ied by sequencing. The e o e, unde ou expe imen al
condi ions, no clinical le el o Fos esis ance can be achie ed by
he o e exp ession o ch omosomal genes, excep mu A.
E ec o mu A o e exp ession on Fos esis ance le el. MICs
o Fos o s ains MG1655(pCA24N) and MG1655(pCA24N-
mu A) we e de e mined by he b o h mic odilu ion me hod as
ecommended by he CLSI (3), excep ha LB b o h was used
ins ead o Muelle -Hin on medium. Table 1 shows how inc eases
in mu A ansc ip ion, caused by ising IPTG concen a ions, p o-
mo e s ong inc eases in he MIC o Fos.
E ec o mu A o e exp ession on g ow h a e. To gain a
quan i a i e insigh in o he i ness cos imposed by his ype o
esis ance, g ow h cu es in he p esence o di e en concen a-
ions o IPTG (3.3, 10, 33, and 100
␮
M) we e eco ded. O e nigh
cul u es we e dilu ed 1:100, and g ow h was esumed o 4 h, wi h
and wi hou di e en concen a ions o IPTG. Cul u es o he
con ol and mu A-ca ying s ains we e hen dilu ed 1:36 in LB
b o h plus IPTG in a inal olume o 180
␮
l pe well in a la -
bo om 96-well pla e. The pla e was co e ed wi h a lid o p e en
e apo a ion and incuba ed a 37°C in a mul iwell luo ime e .
Op ical densi y a 595 nm was eco ded e e y 10 min a e 10 s o
o bi al shaking. Two con ols we e included, s ain MG1655 ha -
bo ing he ec o pCA24N and he s ain exp essing mu A,
MG1655(pCA24N-mu A), wi hou IPTG. Figu e 1A shows ha
he addi ion o IPTG, e en a he highe concen a ion, did no
p oduce any e ec on he g ow h o he con ol s ain in LB b o h.
On he con a y, when he exp ession o mu A was inc eased by
Recei ed 11 No embe 2011 Re u ned o modi ica ion 25 Decembe 2011
Accep ed 18 Feb ua y 2012
Published ahead o p in 27 Feb ua y 2012
Add ess co espondence o Jesús Blázquez, [email p o ec ed].
Copy igh © 2012, Ame ican Socie y o Mic obiology. All Righ s Rese ed.
doi:10.1128/AAC.06122-11
0066-4804/12/$12.00 An imic obial Agen s and Chemo he apy p. 2767–2769 aac.asm.o g 2767
on July 26, 2017 by USE/BCTA.GEN UNIVERSITARIAh p://aac.asm.o g/Downloaded om
he addi ion o di e en concen a ions o IPTG, he g ow h o
MG1655(pCA24N-mu A) was a ec ed (Fig. 1B).
These esul s indica ed ha o e p oduc ion o Mu A con e s a
i ness cos p opo ional o he le el o ansc ip ion (Fig. 2),
which opens he possibili y o e olu iona y adjus men o he
esis ance- i ness adeo . In pa icula , clinical esis ance le els
(32
␮
g/ml) can be achie ed a a low i ness cos (abou 5%, he
esul o in e pola ion in Fig. 2), whe eas he i ness cos o he
commonly ound pe meabili y mu an s (wi h al e ed GlpT and/o
UhpT unc ionali y) has been es ima ed o be ⬃20% (19). A sim-
ple calcula ion can illus a e he meaning o his i ness cos s, in
e ms o how long i will ake o a wild- ype s ain o domina e he
popula ion. Assuming ha bo h wild- ype and mu an popula-
ions g ow exponen ially and excluding de no o mu a ions, i can
be easily shown ha he ime needed o a y he a io o mu an o
wild- ype bac e ia by ac o C(
C
) is equal o log
W
C, whe e wis he
ela i e i ness o he mu an ( a io o he g ow h a es). Calcula -
ing
C
wi h i ness cos s o 5% and 20%, espec i ely, shows ha i
will ake mu A o e exp ession mu an s mo e han ou imes as
long as pe meabili y mu an s o be ou compe ed by he wild ype
in he absence o an ibio ic p essu e. The exac numbe o gene -
a ions depends on C. Fo example, i will ake he wild- ype 51.6
gene a ions o inc ease i s equency om 0.001% o 90% agains
pe meabili y mu an s, whe eas i will ake 224.5 gene a ions
agains o e exp ession mu an s.
Due o he cu en sca ci y o new an ibio ics, Fos has been
p oposed as an al e na i e ea men o in ec ions caused by a
wide a ie y o bac e ia (7). Ou genomewide sc eening has shown
ha o he 5,272 ch omosomal genes es ed, only 1, mu A, is able
o con e clinical le els o esis ance when o e exp essed. E en
TABLE 1 E ec o mu A ansc ip ion le el on os omycin MIC o
s ain MG1655 con aining ei he pCA24N ( ec o alone) o pCA24N-
mu A (exp essing mu A)
IPTG concn (
␮
M)
MIC o os omycin (
␮
g/ml)
pCA24N pCA24N-mu A
014
3.3 1 8
10 1 16
33.3 1 64
100 1 4,096
FIG 1 G ow h cu es a di e en IPTG concen a ions. The alues shown a e a e ages o se en independen expe imen s. Di e en shades o g ay ep esen
di e en concen a ions o IPTG. (A) G ow h o con ol s ain MG1655 ca ying he cloning ec o pCA24N. (B) G ow h o he s ain ca ying mu A-exp essing
plasmid pCA24N-mu A.
FIG 2 T adeo be ween i ness and esis ance. Fi ness and MICs we e es i-
ma ed o s ain MG1655 wi h a plasmid ha bo ing he mu A gene a di e en
le els o induc ion (exp essed as concen a ions o IPTG). The g ow h a e was
es ima ed as he maximum slope o he na u al loga i hm o op ical densi ies
e sus ime. Cu e i ing was done using a eg ession spline me hod (4, 24),
and i ness was calcula ed as he a io o he g ow h a e a each concen a ion
o IPTG o ha wi hou IPTG (16). These measu emen s we e de e mined
om se en independen cul u es, and a single mean ela i e i ness le el was
calcula ed o each condi ion. E o ba s ep esen s anda d de ia ions.
Couce e al.
2768 aac.asm.o g An imic obial Agen s and Chemo he apy
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hough he e ec o o e exp ession o mu A on Fos esis ance has
been desc ibed al eady (9, 16, 19), ou esul s indica e ha no
o he ch omosomal genes can p oduce Fos esis ance by o e ex-
p ession. In addi ion, we show ha o e p oduc ion o Mu A o a
le el high enough o p oduce clinical Fos esis ance exac s a i ness
cos signi ican ly lowe han ha imposed by o he mu a ions
ound in clinical isola es (19).
How likely a e mu an bac e ia o e exp essing mu A o be se-
lec ed in i o? Apa om he i ness cos , he mu a ion a e also
plays a majo ole in de e mining his p obabili y (2). O e exp es-
sion mu an s could easily a ise unde na u al condi ions by in-
c easing RNA polyme ase binding a ini y h ough p omo e mu-
a ion, by dis up ing gene con ol exp ession as desc ibed o
o he esis ance de e minan s (6, 17), o by acqui ing o eign
DNA segmen s ia, o ins ance, inse ion sequences wi h s ong
p omo e ac i i y (5, 14). In any case, u he clinical s udies a e
necessa y o de e mine he ela i e impo ance o his mechanism
o esis ance.
ACKNOWLEDGMENTS
This wo k was suppo ed by g an s PI10/00105 and REIPI RD06/0008
om he Minis e io de Ciencia e Inno ación, Ins i u o de Salud Ca los III
( he la e was co inanced by he Eu opean De elopmen Regional Fund
“A Way o Achie e Eu ope” ERDF); by he Spanish Ne wo k o Resea ch
on In ec ious Diseases (REIPI RD06/0008); and by he PAR p ojec
(241476) om he EU 7 h F amewo k P og amme.
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Fos omycin Resis ance a Low Fi ness Cos
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