I. Lutsar et al.
intracisternal injection of pneumococcal cell walls, or after the first
injection of ampicillin, in experimental meningitis caused by live
Material and methods
Pneumococcal cell wall (1 cm3 of this product was obtained from
2.5 × 109 live organisms) was produced and provided by Dr Elaine
Tuomanen.10 Streptococcus pneumoniae (MIC and MBC of ampicillin,
0.01 mg/L) was originally isolated from a patient with bacterial menin-
gitis. To induce meningitis, 0.2 mL pneumococcal cell wall product, or
104–105 cfu/mL oflive organisms, were inoculated intracisternally.
Meningitis model and treatment
A rabbit meningitis model originally described by Dacey & Sande11
was used. In the first set of experiments, DXM (1 mg/kg) was given
intravenously 30 min before or 1 h after the administration of pneumo-
coccal cell walls to six and seven animals, respectively. In the second set
ofexperiments, 16–18 h afterinoculation ofliveorganisms,animalswere
treated with ampicillin alone (75 mg/kg every 12 h) for 24 h, or with the
before or 1 h after the first ampicillin dose. Each treatment group con-
sisted of 12–13 animals. No treatment was given to control animals.
CSF sample collection and analysis
CSF was collected directly from the cisterna magna under acepromazine
and ketamine anaesthesia before and 2, 4, 6, 12 and 24 h after start of
therapy. For the first four CSF collections, animals were restrained
under anaesthesia in stereotactic frames. Leucocytes were counted in
a Neubauer haemocytometer. Bacterial concentrations in CSF were
measured by plating undiluted and serial dilutions of CSF on sheep
blood agar and incubating in 5% CO2 at 35°C for 24 h. The lower limit
of detection was 10 cfu/mL. The remaining CSF was centrifuged and
the supernatant stored at –70°C. CSF lactate concentrations were meas-
ured by a photocolorimetric assay (Behring Diagnostics Inc, Milton
Keynes, UK). TNF-α concentrations were measured by cytotoxic assay
using L929 tumorigenic murine fibroblasts.12 The standard curve for
theTNF-α assay was linearfrom 40–2500 pg/mL.
Normallydistributeddataarepresentedas mean ±S.D. andnon-normally
distributed data as median and range. Student’s t-test was used for
comparison of parametric data and the Kruskall–Wallis analysis of
variance for non-parametric variables.
Meningitis induced by pneumococcal cell walls
Administration of pneumococcal cell walls resulted in the release of
TNF-α, an influx of leucocytes and increased lactate concentrations
in CSF (Figure 1). The elevation in TNF-α concentrations was pre-
vented by DXM therapy when given 30 min before or 1 h after pneu-
mococcal cell walls. The increase in CSF leucocyte and lactate
given 30 minbefore the cell wall products.
Meningitis induced by live organisms
The addition of DXM to ampicillin therapy resulted in a lower initial
bacterial killing rate compared with ampicillin therapy alone (0.39 ±
0.1 cfu/mL/h versus 0.57 ± 0.12 cfu/mL/h; P = 0.04). The changes in
CSF inflammatory indices were similar in all study groups and were
not influenced bythe co-administrationofDXM(Figure 2).
For further analysis, animals were divided in two groups based on
CSF bacterialconcentrationsat the start oftherapy(≤5.5log10 or≥5.6
log10 cfu/mL) (Figure 3). After the first dose of ampicillin, animals
with high initial bacterial concentrations demonstrated significantly
greater changes in CSF TNF-α [median ∆+683 pg/mL (quartiles
+246 to +758) versus ∆–16.3 pg/mL (quartiles –6 to –29)], white
bloodcells (WBC)[median ∆+2175 cells/mL (quartiles 437to 6987)
versus ∆–325 cells/mL (quartiles –787 to +25)] and lactate [median
∆+14.3 mmol/L (quartiles –7.6 to –14.6) versus ∆–8.5 mmol/L
lowerinitial bacterial concentrations.
Figure 1. Concentrations of WBC (cells/mL; left), TNF-α (pg/mL; middle) and lactate (mmol/L; right) in the CSF. Meningitis was induced by the intracisternal
administration ofpneumococcalcellwalls.Animalsweretreated with DXMgiven 30 min before (open squares)or 1h after (solid squares) administration of cellwalls.
No treatment was given to the control animals (solid triangles). *P = 0.02 versus controls or those treated with DXM 1 h after introduction of cell walls.
Dexamethasone in pneumococcal meningitis
In those with high initial bacterial concentrations (≥5.6 log10 cfu/
mL), DXM therapy prevented elevations in CSF TNF-α [median
∆–184 pg/mL (quartiles –116 to –258) or ∆–114 pg/mL (quartiles –
39 to –175) versus ∆+683 pg/mL (quartiles 246 to +758) with DXM
given 30 min before or 1 h after ampicillin versus without DXM,
respectively; P = 0.02] and lactate concentrations [median ∆–10.6
mmol/L (quartiles 7.6 to 17.4) or ∆–1.5 mmol/L (quartiles –19.7 to –
1.3) versus ∆+14.3 mmol/L (quartiles –7.6 to –14.6) with DXM
given 30 min before or 1 h after ampicillin and without DXM,
respectively; P = 0.01]. These effects were not significantly different
as a result of the timing of DXM administration, although there was a
trend indicating that changes in TNF-αand lactate values were lower
in animals given DXM before ampicillin therapy (Figure 4). The
changes in leucocyte concentrations were not affected by DXM ther-
apy. In animals with low CSF bacterial concentrations (≤5.5 log10
cfu/mL), there were no differences in TNF-α, WBC or lactate con-
centrations between those treated with or without DXM (data not
shown). There was no correlation between changes in TNF-α, leuco-
cyte and lactate concentrations in CSF, and the degree of bacterial
killing after introduction of ampicillin therapy (r = 0.06; r = 0.47 and
r =0.46, respectively; P> 0.05).
In this model of pneumococcal meningitis, we demonstrated that the
antibiotic-induced secondary inflammatory response, as evidenced
Figure 2. Concentrations of WBC (cells/mL), TNF-α (pg/mL), lactate (mmol/
L) and S. pneumoniae (log10 cfu/mL) in CSF in animals with pneumococcal
meningitis. Animals were treated with ampicillin alone (solid triangles), with
DXM given 30 min before (open squares) or 1 h after ampicillin therapy (solid
squares). No treatment was given to the control (open triangles) animals, and
they died after 6–10 h. Error bars demonstrate lower and upper quartile.
*P = 0.04 versus those not receiving DXM therapy.
Figure 4. Concentrations of WBC (cells/mL, left), TNF-α (pg/mL, middle), lactate (mmol/L, right) in CSF in animals with initial CSF bacterial concentrations ≥5.6
log10 cfu/mL. DXM therapy was given 30 min before (open squares) or 1 h after the first dose of ampicillin (solid squares). Control animals (solid triangles) were
treated with ampicillin only. Error bars demonstrate lower and upper quartiles. *P < 0.05 versus those treated with ampicillin and DXM.
Figure 3. Changes in CSF concentrations of WBC (cells/mL), TNF-α (pg/mL),
lactate (mmol/L) and S. pneumoniae (log10 cfu/mL) in pneumococcal meningi-
tis after introduction of ampicillin therapy. The CSF bacterial concentrations at
the introduction of ampicillin therapy were ≤5.5 log10 cfu/mL (open squares) or
≥5.6 log10 cfu/mL (solid squares). Error bars demonstrate lower and upper quar-
tile. *P < 0.05
I. Lutsar et al.
by an elevation of CSF TNF-α and lactate concentrations, occurred
only in animals with high bacterial concentrations before the start
of ampicillin therapy. The increase in CSF lactate and TNF-α con-
centrations were inhibited by adjunctive DXM therapy regardless
of whether it was given 30 min before or 1 h after the first dose of
Liberation of free endotoxin or cell-wall components by cell-wall
active antibiotics has been demonstrated in vitro and in experimental
meningitis.3,4,5,13 This is associated with enhanced inflammation in
the subarachnoid space, as evidenced by an increase in leucocyte,
TNF-αand lactate concentrations.3,14 Ourstudyshowed that the anti-
biotic-induced inflammatory burst did not occur in all animals and
was seen only in those with greater CSF bacterial concentrations
(≥5.6 log10 cfu/mL). Although this has been intimated previously,
data were not provided.14 In experimental meningitis, the magnitude
inoculated cell walls10 and thus it is not surprising that animals with
higher bacterial counts demonstrate a more pronounced host inflam-
matory response after antibiotic therapy.
There is concern that rapid bacterial killing by antibiotics could
result in an enhanced inflammatory response and a worsening of the
clinical outcome in meningitis.15,16 The results of this and previous
studies, however, do not support these speculations. On the contrary,
clinical and experimental studies have demonstrated that rapid effec-
tive bactericidal therapy protects against the development of deaf-
ness and other neurological disabilities.17,18 Moreover, this and
previous studies4 found no correlation between CSF inflammatory
markers and the magnitude of bacterial killing. Uncontrolled bac-
terial growth eventually results in a much greater release of cell wall
components—and consequent enhancement of the inflammatory
response—than that induced by antibacterial therapy.4,13 These find-
ings suggest that early antibacterial therapy and rapid clearance of
bacteria from CSF outweigh the potential adverse effects caused by
the antibiotic-induced inflammatoryburst.
DXM, as an adjunct to antibacterial therapy in bacterial meningi-
tis, has been shown to be beneficial in H. influenzae meningitis in
children.6 In pneumococcal meningitis, however, its modulatory
effect was uncertain because of the relatively small numbers of chil-
dren enrolled in prospective controlled trials.19 In adults with pneu-
mococcal meningitis, an unfavourable outcome was seen in 26% of
receiving placebo.9 The results of our study clearly supported previ-
ous findingsbyTuomanenet al.8 and showed that,similarto H. influ-
enzae meningitis, DXM therapy prevents the antibiotic-induced
release ofTNF-αand lactateconcentrationsinCSF inpneumococcal
Early institution of DXM therapy has been suggested because of
its delayed onset of action.20 In our study, the timing of DXM
appeared to be critical only in meningitis induced by the pneumo-
coccal cell wall. Inmeningitis induced bylive microorganisms, how-
ever, once CSF inflammation was established, administration of
DXM before the dose of ampicillin did not appear to have a signifi-
cantlygreater salutaryeffectthan1hafter ampicillin;DXM adminis-
tration that was delayed further was not assessed. These results
contrast with those obtained in experimental H. influenzae meningi-
tis, where the antibiotic-induced inflammatory response was modu-
lated only if DXM was given before or simultaneously with
ceftriaxonetherapy.3 The effectivenessofearly DXM administration
was also highlighted ina recent meta-analysis of 10 randomized con-
trolled clinical trials.6
At the time of diagnosis, ∼40% of patients with pneumococcal
meningitishave CSFbacterial concentrations<106 cfu/mL21,andour
findings suggest that antibiotic-inducedenhancedinflammationmay
not occur in such patients. Whether these patients benefit from DXM
therapy, and how they can be identified at diagnosis, requires further
clarification. Detection of bacteria in Gram-stained specimens of
CSF maybeuseful inidentifyingpatientswithlarge bacterialloads.22
One possible detrimental effect of DXM therapy, also demon-
strated in this study, is decreased bacterial clearance from CSF.23,24
in decreased concentrations of hydrophilic antibiotics in the CSF.25
by CSF leucocytes.26The clinical relevance of these findings, how-
ever, has not been demonstrated.6,9
In this model of pneumococcal meningitis, CSF bacterial concen-
trations at the start of therapy appeared to be more important than the
timing ofDXMtherapy in influencing the antibiotic-induced inflam-
matory response. It is likely that there is a time beyond which DXM
loses its effectiveness, but this point has not been clearly defined.
I. Lutsar was a recipient of a fellowship award from the European
Society for Paediatric Infectious Diseases, supported by Lederle-
meeting of Infectious Diseases Society of America (IDSA), San
Francisco. The study has followed animal experimentation guide-
lines and was approved by the Institutional Animal Care and
Research Advisory Committee of the University ofTexas.
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