small colony variants - new review by Proctor - Penny

[Guest guest]

The new review is here:

http://www.nature.com/nrmicro/journal/v4/n4/full/nrmicro1384.html

Penny, theres a bit on S. aureus small-colony variants in

osteomyelitis midway thru this post.

I'm not reading very well (lil headache), but I think all the SCVs

discussed in this passage may have a similar phenotype:

" In a recent study, NRK-49F fibroblasts were infected with S.

typhimurium, and 103 SCVs were isolated58. Three SCVs that were able

to persist intracellularly were found to have defects in hemL, aroD

and ldh. HemL and AroD are involved in haem biosynthesis and

menadione biosynthesis, respectively, in Salmonella serovars. These

SCVs were also found to induce less cell lysis than their wild-type

counterparts. Additionally, although the SCVs were less virulent than

their wild-type counterparts, they persisted better in BALB/c mice58.

These data are reminiscent of the phenotype and clinical course of

staphylococcal SCVs, and they indicate that the SCV phenotype might

be a general mechanism for bacterial persistence in mammals. "

The study cited is this one:

Selection of small-colony variants of Salmonella enterica serovar

typhimurium in nonphagocytic eucaryotic cells.

Infect Immun. 2003 Jul;71(7):3690-8.

PMID: 12819049

Thus, antimicrobials aside, it may be that hypometabolic bacteria

enjoy resistance to host defenses. This investigation by Proctor

suggests the same:

J Infect Dis. 1994 Oct;170(4):1033-7.

Gentamicin-resistant menadione and hemin auxotrophic Staphylococcus

aureus persist within cultured endothelial cells.

PMID: 7930701

According to a study by the same salmonellologists, it appears that

the persistant slow-growing state of salmonella in fibroblasts is

salmonella-directed, a function of pathogenicity-regulating gene

products:

Salmonella enterica serovar Typhimurium response involved in

attenuation of pathogen intracellular proliferation.

Infect Immun. 2001 Oct;69(10):6463-74.

PMID: 11553591 [PubMed - indexed for MEDLINE]

Of general interest (back to Proctors new review):

" Interestingly, keratinocytes with internalized SCVs had a healthy

appearance 48 hours after infection, whereas cells infected by the

corresponding normal S. aureus underwent apoptotic or necrotic cell

death6. A similar observation was made in another study in which the

parent strain was highly destructive to cultured endothelial cells,

whereas SCVs did not damage these cells59. Taken together, these data

indicate that the SCV phenotype might be one of the survival

strategies that S. aureus (and perhaps also S. typhimurium) uses for

optimal internalization and survival in the host. "

Penny - Proctor's new review again:

" The incidence of SCVs in clinical specimens has been found to range

from 1% to more than 30% in different studies. S. aureus SCVs were

found in 1% of 1,110 isolates in a general microbiology laboratory23.

Analysis of sputa from 72 patients with [cystic fibrosis] has shown

that more than 70% (52 of 72) are chronically colonized with S.

aureus, and of these samples, 46% (24 of 52) contained SCVs41. In a

study of ***S. aureus recovered from bone specimens or deep-tissue

aspirates of patients with osteomyelitis***, S. aureus SCVs were

found in 29% of patients (4 of 14)33. Finally, three N. gonorrhoeae

SCVs were found in 135 cervical specimens (0.02%)68. So, SCVs are not

rare, but they can be difficult to recover (see Box 1 for details on

how SCVs can be identified in the clinical microbiology laboratory). "

A long bit on osteomyelitis:

" Osteomyelitis. Several investigators have shown that SCVs can often

be recovered from cultures of normal S. aureus strains that have been

exposed to gentamicin or other aminoglycosides24, 47, 59, 62, 63, 65,

66, 93, 94, 95. Beads containing gentamicin are used as an adjunct to

systemic antibiotic therapy and debridement to treat patients with

osteomyelitis. The beads release the aminoglycoside slowly (over

weeks to months), providing a sustained local concentration of the

antimicrobial agent. To examine whether the slow release of

gentamicin into the local environment is an efficient way to select

for SCVs in vivo, a case–control study was initiated33. Bone

specimens or deep-tissue aspirates from patients who were suspected

to have osteomyelitis were screened for SCVs, and only those patients

for whom S. aureus was recovered from at least one specimen were

included in the study. Fourteen patients who carried S. aureus and

had clinical signs of chronic osteomyelitis were found in an 18-month

period. Menadione- or haemin-auxotrophic S. aureus SCVs were

recovered only from those patients who had previously been treated

with gentamicin-containing beads. Large and small colony types were

recovered from three of these patients, and therapy failed despite

using antimicrobials with in vitro activity against these isolates.

By contrast, ten other patients with normal S. aureus (and no

previous placement of gentamicin-containing beads) had no relapses of

osteomyelitis within more than 1 year of primary diagnosis after

active antibiotics were given. No other important differences between

the two groups were detected in a clinical evaluation. SmaI digests

of the whole bacterial DNA of all isolates from each patient were

typed by pulsed-field gel electrophoresis, and despite large

phenotypic differences, the isolates were found to be clonal. The

MICs for gentamicin were up to 32-fold higher for the SCVs (1 g per

ml) than for the parent strain (less than 0.031 g per ml), whereas no

differences were found in susceptibilities to other antimicrobial

agents.

" In a follow-up study, S. aureus SCVs were again often recovered

following previous placement of gentamicin-containing beads (C.v.E.,

R.A.P. and G.P., unpublished observations). However, SCVs were also

cultivated from specimens taken from patients who had no previous

placement of gentamicin-containing beads, and S. aureus with a normal

phenotype were also cultivated following local prophylaxis with

gentamicin-containing beads, without the simultaneous isolation of

SCVs. Nevertheless, although the patient milieu is clearly complex,

the results provide evidence that gentamicin-containing bead

placement might be an efficient way to select in vivo for S. aureus

SCVs33. "

God, I love this crap. What a relief to be out of school for a while.