Evaluation of Intraspecies Interference Due to agr Polymorphism in Staphylococcus Aureus during Infection and Colonization
Christiane Goerke
Mirjam K ummel
Klaus Dietz
Christiane Wolz
Instituts for
Allgemeine Hygiene und Umwelthygiene
Medizinische Biometrie
Universitat Tubingen
Tubingen
Germany
In Staphylococcus aureus infection, intraspecies cross-inhibition mediated by the regulatory system agr may lead to the exclusion of heterologous strains at the site of infection or colonization. We analyzed consecutive and cocolonizing strains (classified as different clones by pulsed-field gel electrophoresis) from patients with cystic fibrosis (CF) and healthy individuals. Strain replacement was accompanied by a change in the agr group in 80% of the patients with CF and in 63% of the healthy individuals. Cocolonizing strains from patients with CF were shown to belong to interfering agr groups in 6 of 10 cases. In contrast, in healthy individuals, cocolonization of the nares with strains of interfering agr groups was rarely observed. agr polymorphism has no impact on the colonization dynamics of S. aureus in patients with CF but may influence nasal colonization in health individuals.
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The major human pathogen Staphylococcus aureus
asymptomatically colonizes the anterior nares of
humans but also causes a wide spectrum of diseases [1].
S. aureus infections are probably established via the
coordinated synthesis of extracellular and cell-bound
virulence factors [2]. The expression of most virulence
factors is controlled by global regulators, such as agr.
The agr locus, which consists of 5 genes (agrA, agrC,
agrD, agrB, and hld), codes for 2 divergent transcripts
(RNAII and RNAIII) initiated by 2 distinct promoters
(P2 and P3). RNAIII is the effector molecule, exhibiting
a positive regulatory effect on extracellular proteins,
such as a-hemolysin, and a negative regulatory effect
on protein A and fibronectin-binding proteins [36].
agrA shows sequence homology to the response
regulator, whereas agrC corresponds to the histidine protein
kinase signal transducer [7] of the classic 2-component
regulatory system [8]. agrD encodes an extracellular
octapeptide, known as the autoinducing peptide (AIP),
which operates in a quorum-sensing system [9, 10],
thus explaining the growth-phasedependent
expression of agr-regulated genes. agrC binds the AIP [11]
and may subsequently phosphorylate AgrA. P2 and P3
are both thought to be autocatalytically activated by
phosphorylated AgrA [7]. The sequence of the AIP and
its receptor, agrC, is highly variable, resulting in at least
4 specificity groups within S. aureus [12, 13]. AIPs
activate their specific receptors while inhibiting activation
of AgrC in strains of other specificity groups. This
inhibition is a form of bacterial interference that does
not result in mutual growth inhibition but, instead, in
the blocking of virulence gene expression. This
phenomenon has been discussed as a new bacterial strategy
for excluding other strains of the same species from the
infection or colonization site [12]. However, such
intrastrain interference during colonization and infection
has never been actually demonstrated in the human
host. Surveys of agr group distribution in naturally
occurring S. aureus strains have been done mainly with
methicillin-resistant S. aureus (MRSA) [14, 15] and
with strains isolated from various infections sites [16,
17]. Information on agr group distribution of nasal
carrier strains in the community is limited [18].
We wanted to analyze whether intraspecies
cross-inhibition of agr expression leads to the exclusion of heterologous
strains at the site of colonization or infection. For this purpose,
agr polymorphism in consecutive and cocolonizing S. aureus
strains was assessed. We compared S. aureus isolates obtained
from patients with chronic infection (lung infection in patients
with cystic fibrosis [CF]) with isolates from nose colonization in
healthy individuals in the community. The prevalence of agr
group IIV strains also was determined in intubated patients
from an intensive care unit (ICU) and in patients with chronic
wounds.
MATERIALS AND METHODS
Strain collection. S. aureus isolates were collected from 133
individuals: 35 nonhospitalized patients with CF (sputum
samples), 30 intubated patients in an ICU (tracheal secretions), 31
patients from an out-patient wound consultation clinic (wound
swabs), and 37 healthy control subjects (nose swabs). Patients
with CF attended the CF center in Tu bingen quarterly for
regular check-ups. Antibiotics were prescribed according to the
standard guidelines of the center. Patients in the ICU were
admitted to this ward after major surgeries. All had received
prophylactic antibiotic treatment. Tracheal secretions were
obtained 25 days after intubation. The category of wound
patients mainly consisted of patients with diabetes mellitus. In all
but 2 cases, the S. aureuspositive wound swabs were not
accompanied by inflammatory signs, and no antibiotic treatment
was applied. The healthy individuals included family members
from a study with CF and non-C (...truncated)