BJU International (2002), 89, 55±60
The migration of Proteus mirabilis and other urinary tract
pathogens over Foley catheters
N . S A B B U B A , G . HU G H E S and D . J . S T I C K L E R
Cardiff School of Biosciences, Cardiff University, Cardiff, Wales, UK
Objective To examine the ability of organisms that
infect the catheterized urinary tract to migrate over
the surfaces of Foley catheters.
Materials and methods In a simple laboratory model,
organisms were challenged to migrate across sections
of hydrogel-coated latex, hydrogel/silver-coated
latex, silicone-coated latex and all-silicone catheters.
The sections (1 cm long) were placed as bridges in
channels between blocks of agar and the test
organisms inoculated onto the agar adjacent to
one side of each bridge. The plates were incubated
at 37uC for 24 h and examined for growth of the
test organisms on the agar on the other side of the
bridges. A collection of swarming, swimming and
nonmotile species were tested in the model. The
relative mobilities of the test organisms were expressed
as migration indices, calculated as the percentage of
tests in which bacterial migration was observed over
each type of catheter bridge.
Results The swarmer cells of Proteus mirabilis and
P. vulgaris migrated successfully (migration indices
of 73±100) over all four types of catheter. The
migration index of Serratia marcescens swarmers
was reduced to 33 over the silver-coated catheters,
but these cells crossed over the other catheter
surfaces with ease (indices of 100). Pseudomonas
aeruginosa was the most mobile of the swimming,
non-swarming organisms with indices of 70±22,
but this group was less capable of migration than
the swarmers. Indices were 0±33 for nonmotile
organisms. The mean migration indices for the nine
species for each type of catheter were 57 (hydrogelcoated latex), 49 (silver/hydrogel-coated latex),
41 (silicone-coated latex) and 35 (all-silicone). The
swarmer cells of P. mirabilis moved through populations of Escherichia coli, Klebsiella pneumoniae,
Staphylococcus aureus and Enterococcus faecalis, and
then migrated over sections of hydrogel-coated
latex catheters with little or no reduction in migration index. They were also capable of transporting
the nonmotile cells of K. pneumoniae and S. aureus
over the catheters. The migration index of
P. mirabilis swarmers was substantially reduced in
the presence of Ps. aeruginosa and S. marcescens.
Conclusions Hydrogel coatings facilitate the migration
of urinary tract pathogens over catheter surfaces.
With the exception of S. marcescens, the incorporation of silver into the hydrogel did not inhibit
migration. Swarmer cells were particularly effective
at moving over catheters and P. mirabilis swarmers
were also capable of transporting other species.
This suggests that inhibitors of swarming could be
useful in controlling catheter-associated infection
and the complications resulting from the spread of
bacterial bio®lm over catheters.
Keywords catheter-associated infection, urinary tract
pathogens, bacterial migration, catheter colonization
Introduction
patients undergoing this form of bladder management
that catheter-associated UTI is the most common of
the infections acquired in hospitals and other
healthcare facilities [2].
Clinical studies [3,4] indicate that bacteria can
initiate the infection by migrating from the urethral
meatus-catheter junction along the external surface
of the catheter into the bladder. Few laboratory studies
have examined the ability of urinary tract pathogens
to migrate over the various types of catheters that
are in clinical use. Stickler and Hughes [5] recently
reported that Proteus mirabilis, a particularly important
Indwelling urinary catheters are used in enormous
numbers in modern medicine in both hospital and
community care [1]. They constitute a convenient
way to manage the problems of urinary retention and
incontinence, but unfortunately they also provide access
for bacteria from a contaminated external environment
into a vulnerable body cavity. Consequently they are
frequently associated with UTI. There are so many
Accepted for publication 1 October 2001
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2002 BJU International
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56
N. SABBUBA et al.
pathogen of the catheterized urinary tract, could
swarm rapidly over all of the major types of urethral
catheters. In this paper we present the results of
observations on (a) the ability of a range of organisms
that commonly infect the catheterized urinary tract
to migrate over catheter surfaces and (b) the migration
of P. mirabilis over hydrogel-coated catheters in the
presence of other species.
Materials and methods
To assess the migration of bacteria over catheter sections,
the strains of bacteria used were P. mirabilis NSM42,
P. vulgaris NSM19, Serratia marcescens NSM51,
Escherichia coli NSM36, Pseudomonas aeruginosa
NSM35, Providencia stuartii NSM71, Klebsiella pneumoniae
NSM48, Staphylococcus aureus NSM49 and Enterococcus
faecalis NSM40; all were recent clinical isolates from
catheter-associated UTI. They were stored at x70uC
in 5% (v/v) glycerol. Before experimental work they
were subcultured onto CLED Agar (Oxoid Ltd.
Basingstoke, UK). In the test method, plates containing
17 mL of tryptone soya agar (TSA, Oxoid) were dried
at ambient temperature for 18 h. Channels (8.5 mm
wide) were cut across their centres and an additional
channel cut at right angles as shown in Fig. 1a.
After drying for a further 3 h, three aliquots (10 mL)
of 4-h cultures of test strains in tryptone soya broth
(TSB, Oxoid) were inoculated at the edge of the
central channel of each plate. After the inocula had
dried into the agar, sections (1.0 cm long) of #14
catheters (all-silicone catheters and latex catheters
coated with silicone, hydrogel or silver/hydrogel,
Bard Ltd. Crawley, UK) were placed as bridges across
the central channels adjacent to two of the inocula.
As a control, the third inoculum was not provided
with a bridge (Fig. 1a). The exposed ends of the
balloon-in¯ation line of the catheter sections were
sealed with Silicoset 151 (Ambersil Ltd, Bridgewater,
UK). The plates were incubated at 37uC for 24 h and
examined for the growth of the test organisms on the
un-inoculated halves of the plates. When growth
appeared on the far side of the agar adjacent to a
catheter section but not on the control section of the
plate where no catheter section was placed, it was
concluded that the test strain had migrated across the
catheter surface.
A glycerol-peptone agar containing peptone 5 g/L,
glycerol 1% v/v and agar 0.65% w/v [6] was used in
the tests with S. marcesens to induce swarming. To
examine the migration of swimming cells (rather than
swarmers) TSA was used for S. marcesens and a
minimal-salts agar [7] for P. mirabilis.
To assess the migration of P. mirabilis over hydrogelcoated catheters in the presence of other species,
aliquots (10 mL) of cultures of the test organisms
that had been grown in TSB for 4 h at 37uC were
inoculated adjacent to the edge of central channels
that had been cut in TSA plates. When these drops
had dried into the agar, inocula of P. mirabilis NSM42
were then placed 1 cm from the central channel, in line
with the inoculum of the other species. The inocula
were allowed to dry for 15 min before hydrogel-coated
latex catheter sections (1 cm) were placed across the
gap adjacent to test species inocula. This method
ensured that to migrate over the catheter sections,
Fig. 1. a, The model in which the migration of organisms over
1 cm sections of catheters was tested. The plates were inoculated
with (10 mL) of 4-h TSB cultures; b, shows the swarming of
P. vulgaris over silicone (top) and hydrogel-coated latex (bottom)
sections after incubation at 37uC for 24 h.
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2002 BJU International 89, 55±60
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Table 1 The migration of urinary tract pathogens over different types of catheters
Migration index² over each type of catheter
Test species
Swarming cells
Proteus mirabilis
Proteus vulgaris
Serratia marcescens
Swimming cells
Escherichia coli
Pseudomonas aeruginosa
Providencia stuartii
Proteus mirabilis*
Serratia marcescens*
Non-motile cells
Klebsiella pneumoniae
Staphylococcus aureus
Enterococcus faecalis
Mean migration index
Hydrogel-coated latex
Silver/hydrogel-coated latex
Silicone-coated latex
All-silicone
100
100
100
100
100
33
87
100
100
95
73
100
20
70
47
100
37
25
67
33
96
33
16
67
25
0
25
19
22
27
0
30
22
33
0
57
25
25
0
49
25
8
0
41
11
8
0
35
²Calculated as the percentage of tests in which there was bacterial migration over the catheter bridges. A minimum of 12 tests was conducted in each
case. *Indicates that in these cases organisms were tested on agars which did not permit swarming.
the P. mirabilis swarmers had ®rst to move through
populations of the test species. After incubation at
37uC for 18 h the plates were examined for swarming
on the other side of the catheter bridge. Scrapings
were also taken from the catheter exit sites and spread
onto CLED agar (Oxoid) to determine whether the test
species had also migrated over the catheter sections.
Results
The results of experiments which examined the ability
of nine species commonly responsible for catheterassociated infection to migrate over the four types of
catheters are presented in Table 1. The migration
indices were calculated as the percentage of tests in
which bacterial migration was observed over each
type of catheter bridge. The data show that both
species of Proteus migrated successfully across the
sections of all four types of catheter (Fig. 1b). The
migration of S. marcescens across the silver-coated
catheters was reduced, but it swarmed well over the
other three types of sections. Ps. aeruginosa was the
most mobile of the swimming, non-swarming
organisms, but this group was less capable of migration
than the swarmers. Non-motile organisms usually
failed to spread over the catheters. In general, migration
was more apparent over the hydrogel-coated catheters.
The results presented in Table 2 show that
P. mirabilis was able to swarm through populations
of Ent. faecalis, K. pneumoniae and Staph. aureus (Fig. 2b)
and then migrate freely over hydrogel-coated catheters
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2002 BJU International 89, 55±60
Table 2 The ability of P. mirabilis NSM42 to swarm through inocula
of other species and over hydrogel-coated catheters
Test species
Reduction (%) in
migration index of P. mirabilis*
Escherichia coli
Pseudomonas aeruginosa
Serratia marcescens
Klebsiella pneumoniae
Staphylococcus aureus
Enterococcus faecalis
12.5
87.5
87.5
12.5
0
0
*Calculated as the difference in the migration indices of P. mirabilis in
the presence and absence of each test species; in each case there were
eight replicate tests.
with little or no reduction in migration index.
However, inocula of Ps. aeruginosa or S. marcescens,
located between P. mirabilis and the sections substantially reduced the migration index of the swarmer cells
over the hydrogel-coated catheter surface (Fig. 2a).
Discussion
P. mirabilis complicates the care of many patients undergoing long-term indwelling bladder catheterization; it is
commonly responsible for the encrustation and
blockage of the catheters [8±10]. It colonizes the
catheter surfaces, forming bio®lm communities
embedded in a polysaccharide matrix. The bacterial
urease enzyme generates ammonia and elevates the
pH of the urine and the bio®lm. Under these conditions
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N. SABBUBA et al.
Fig. 2. a, The swarming of P. mirabilis over a hydrogel-coated
latex catheter section and the inhibition of this migration by
Ps. aeruginosa. The inocula of P. mirabilis were placed 1 cm away
from the edge of the agar channels. The test strain of Ps. aeruginosa
was inoculated directly on the edge of the agar. After incubation
at 37uC for 24 h, while the P. mirabilis migrated across the
section in the centre of the plate, it failed to migrate through a
population of Ps. aeruginosa. Growth and pigment production by
Ps. aeruginosa can be seen on the un-inoculated block of agar,
indicating that the swimming cells of this species can migrate
across hydrogel-coated latex. b, The ability of P. mirabilis to migrate
through a population of Staph. aureus growing as a macro-colony on
the edge of the agar (centre sector) and then over a section of
hydrogel-coated latex catheter.
struvite (magnesium ammonium phosphate) and
apatite (calcium phosphate) are formed and become
trapped in the organic matrix which surrounds the
cells [11,12]. The continued development of this
crystalline bio®lm can eventually completely block the
catheter lumen [13,14]. The crystalline deposits can be
hard and abrasive, and can traumatize the bladder
mucosa and urethra on catheter withdrawal. As the
bio®lm accumulates it can obstruct the ¯ow of urine
through the catheter, causing either incontinence from
leakage or painful distension of the bladder by urinary
retention.
P. mirabilis can transform from small swimming bacilli
into elongated, highly ¯agellated swarmer cells when
it contacts a surface, and this is accompanied by a
substantial increase in the production of urease [15,16].
Stickler and Hughes [5], using the simple in vitro model
shown in Fig. 1a, found that P. mirabilis could move
rapidly over catheter surfaces, swarming over 1 cm
sections of all-silicone catheter in just 13 h. Migration
was even more rapid over 1 cm sections of silicone-coated
(12 h), hydrogel-coated (8 h) and hydrogel/silver-coated
(8 h) latex catheters. These observations led to the suggestion that swarming facilitates the spread of crystalline
bio®lm over catheter surfaces [5].
The results presented in Table 1 indicate that
P. vulgaris is also capable of swarming rapidly over
the four types of catheters. S. marcescens was less able
to swarm over the silver-coated catheters, only migrating across 33% of the test sections in the 24 h incubation
period. However, it swarmed successfully over all the
sections of the three other catheter types.
When the migration of P. mirabilis and S. marcescens
was tested under conditions in which they did not
swarm, these organisms had a markedly reduced
ability to migrate. P. mirabilis swimmer cells were still
capable of migration over the hydrogel and hydrogel/
silver-coated latex catheters, but they failed to move
over the silicone-coated latex and all-silicone surfaces.
Under non-swarming conditions the ability of
S. marcescens to migrate over the hydrogel and
silicone surfaces was also greatly reduced. Of the
swimming but non-swarming organisms, Ps. aeruginosa
was the most mobile; E. coli and Pv. stuartii generally
showed a limited ability to get across the sections,
particularly the silicone surfaces. The non-motile
species usually failed to migrate across the catheters.
From the results presented in Table 1 it is also clear
that the hydrogel-coating of catheters facilitates
bacterial migration along their surfaces.
Darouiche et al. [17] used an in vitro model of the
catheterized bladder to investigate the movement of
urinary tract pathogens over all-silicone catheters.
Ps. aeruginosa was the most mobile of the species
tested, taking 12 days to migrate along a 10-cm length
of catheter into the bladder. E. coli took 20 days and
the non-motile species K. pneumoniae and Ent. faecalis
took 16 and 27 days, respectively, to reach the bladder.
In view of its importance as a pathogen of catheterized
urinary tract it is surprising that P. mirabilis was not
included in this study.
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M I G RA T I O N O F P R O T E US MI R A B I L I S A N D O T H E R PA T H O G E N S O V E R F O L E Y CA T H E T E R S
The periurethral skin of catheterized patients is
commonly colonized by mixed populations of organisms
[18]. Tests were therefore devised to examine the
mobility of P. mirabilis along catheters in the presence
of other species. In the simple model, the swarmer cells
of P. mirabilis were challenged to migrate through
populations of other species. From the results presented
in Table 2 it is clear that the swarmer cells were
capable of negotiating their way through dense populations of E. coli, K. pneumoniae, Staph. aureus and Ent.
faecalis, and then migrating over the catheters. Scrapings
taken from the agar adjacent to the catheter exit sites
showed that the swarmers were also consistently
capable of transporting K. pneumoniae and Staph. aureus
(but not the other species) over the catheter surface.
However, populations of Ps. aeruginosa and S. marcescens
had an inhibitory effect on the migration of P. mirabilis
(Fig. 2).
The mechanism of these antagonistic effects is
interesting; perhaps these two species secrete products
that are inhibitory to P. mirabilis. They may in some way
be able to interfere with the regulatory system which
controls the transition of Proteus swimmer cells into
swarmers [19].
Scanning electron microscopy showed that P. mirabilis
moved over all-silicone catheters as discrete rafts of
swarmer cells [5]. The results presented in Table 2
suggest that these rafts can also transport other bacteria.
However, it appears to be a selective system, allowing
co-migration with Staph. aureus and K. pneumoniae, but
not E. coli or Ent. faecalis.
In conclusion, these results show that urinary tract
pathogens migrate most easily over hydrogel-coated
catheters. The incorporation of silver into the hydrogel
had little effect on the ability of most species to migrate
over the surface. Swarmer cells of P. mirabilis, P. vulgaris
and S. marcescens were the most effective at migration.
P. mirabilis swarmers were also shown to be capable of
transporting some non-motile organisms over catheters.
Swarmer cells of Proteus with their substantially
increased levels of urease (the driving force of catheter
encrustation) and their ability to migrate rapidly
over catheters, could well facilitate the spread of crystalline catheter-blocking bio®lm and initiate ascending
infection of the catheterized urinary tract. This
hypothesis leads to the prediction that inhibitors of
bacterial swarming could control infection and the
complications caused by catheter encrustation.
Acknowledgements
G. Hughes was supported by a postgraduate research
studentship funded jointly by Cardiff University and
Biocompatibles Ltd, Farnham, UK.
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Correspondence: D.J. Stickler, Cardiff School of Biosciences,
Cardiff University, Cardiff CF1 3TL, Wales, UK.
e-mail: [email protected]
Authors
N. Sabbuba, BSc, Post-graduate research student.
G. Hughes, BSc, Post-graduate research student.
Abbreviations: TSA, tryptone soya agar; TSB, tryptone
soya broth.
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2002 BJU International 89, 55±60