Transcript susceptibility testing of anaerobes

Clinical data to support the
interpretation of susceptibility
testing of anaerobes
Robin Howe
'Mixed anaerobes
sensitive to metronidazole'
(common practice in UK)
Pus from cerebral abscess
Primary plate after 5 days
AnO2 incubation
Courtesy Val Hall
? But would we report:'Mixed aerobes sensitive to a cephalosporin'
'Fungus present, try Athlete's Foot powder'
'Virus detected, have a hot whisky and lemon'
And would we test them like this?…..
Courtesy Val Hall
'S.O.P.' for susceptibility testing
of anaerobes
• Agar: Any (selective or non-selective)
• Inoculum: Direct sample, ?mixed, not standardised
• Antimicrobial agents: metronidazole
• Incubation: On bench till 5pm, then AnO2 18-72hrs
• Controls: None
• Interpretation:
– Any sized zone = mixed anaerobes, sens to MZ
– No zone = no anaerobes isolated
– Colonies within zone = aerobes
Courtesy Val Hall
Why is susceptibility testing of anaerobes
not generally used in clinical decisionmaking?
• Technical issues with testing
– Slow growth
– Lack of consensus regarding agar/method
• Anaerobic susceptibility patterns
– Predictable
– Unchanging over years
• Limited data to correlate in vitro results with
outcome
– Infections often polymicrobial
– Outcome affected by multiple factors (eg surgery)
Are resistance rates predictable?
Does in vitro resistance correlate with an
identifiable resistance mechanism?
B. fragilis
Prevotella
Porphyromonas
Peptostreptococcus
Fusobacterium
C. difficile
% Clindamycin Resistance
1987
2000
3
26-44
<10
67
Resistance mechanism
ermF, ermG, ermS
ermF
ermZ, ermB, ermBZ
Hecht (2004) CID39: 92
Are resistance rates predictable?
Does in vitro resistance correlate with an
identifiable resistance mechanism?
% Resistance to -lactams
-lactam/
Penicillin Cephamycins
-lactamase
inhibitors
5
8-22
>97
B. fragilis
83
Prevotella
9
Fusobacterium
0-5
21
Porphyromonas
6
Peptostreptococcus
Non-perfringens
0-5
0-88
Clostridia
0
0-10?
C. perfringens
Carbapenems
<0.2
0-5
0
Hecht (2004) CID39: 92
• Baquero (1992)
– 10% C. perfringens in
Spain resistant to pen
(MIC >0.5 mg/L)
BSAC BP 0.12 mg/L
Are resistance rates predictable?
Does in vitro resistance correlate with an
identifiable resistance mechanism?
% Resistance to -lactams
-lactam/
Carbapenems
Penicillin Cephamycins
-lactamase
inhibitors
<0.2
5
8-22
>97
B. fragilis
83
Prevotella
•B.
fragilis
9
Fusobacterium
0-5
0-5
•Class 2e cephalosporinase
21
Porphyromonas
•Altered PBPs
Peptostreptococcus
•V. common 6
•Rare
Non-perfringens
0-5
•Inhibited by0-88
clavulanate etc
•Porin
loss
Clostridia
•Cephamycinases
0
0 •Reported
0-10?
C. perfringens
•Uncommon
•cepA, cfxA
•Zinc metallo--lactamase
•cfiA, ccrA
•Present in ~4% - not usually expressed
Are resistance rates predictable?
Does in vitro resistance correlate with an
identifiable resistance mechanism?
• Reduced susceptibility to
metronidazole
– Common in
• Propionibacteria & actinomycoses
– Rare in
• B. fragilis
– nim genes
• C.difficile
REFERRALS TO ARU
•Bacteroides spp. n=78
(5% of all Bacteroides)
• Clostridium paraputrificum
n=5 (4%)
• Clostridium ramosum
n=3 (1%)
nim genes
• Nim = nitro-imidazole reductase
• Types A – G found in Bacteroides spp.
• Detected by PCR-RFLP
• Chromosomal / plasmid-borne
• Absent from some MZ resistant orgs
• Probable alternative mechanisms
• High level MZ resistance can be induced in
some nim-containing strains
Courtesy Val Hall
• nim genes identified in 2% of 1,502 B. fragilis
from 19 European countries
• nim genes identified in 2% of 1,502 B. fragilis
from 19 European countries
The role of anti-anaerobic therapy
Animal studies
• Rat model of secondary peritonitis
– Pooled faecal emulsion intraperitoneally
– initially E. coli predominates with often fatal
bacteraemia
– If survive  abscesses with B. fragilis
– Early gentamicin  no bacteraemia but late abscesses
– Early clindamycin  no effect on bacteraemic
mortality but reduced late abscesses in survivors
Onderdonk et al (1974) Infect Immun 10:1256
Animal studies
?synergistic infection
• IP injection of mixtures of three orgs
FOX
8
16
8
8
2
B. fragilis
B. thetaiotaomicron
B. ovatus
B. distasonis
E. coli
No
deaths
E. coli
B. fragilis
B. theta
Saline
Cefoxitin
Cefotetan
Amp/sul
13
4
0
7
Cefotetan
8
64
128
128
0.5
No with
abscesses in
survivors
17/17
3/26
3/30
4/23
Amp/sul
2
4
2
4
8
Bacterial counts
B.
B.
E. coli
fragilis
theta
7.4
8.7
8.3
4.8
4.7
3.9
4.5
4.0
3.6
5.4
3.2
2.8
Brook (1994) JAC 34: 791
Reports of clinical failure
associated with resistance
• Penicillin vs C. perfringens
– NIL
• Metronidazole vs Bacteroides spp
– YES
Rotimi et al (1999)CMI 5: 166
• 3 case reports
– 75 yrs female
•
•
•
•
post op Hartmann’s treated with CAZ/MTZ
Readmitted with paracolic abscess
B. frag isolated (MTZ MIC >32 mg/L)
Cured with drainage + IMI
– 40 yrs male
• Gangrenous appendix (mixed B. frag/E. coli/Pseudomonas)
• CXM/MTZ started
• Day 5 - Wound infection
– B. frag + B. ovatus (MTZ MIC >32mg/L)
– Cured with co-amox
– 37 yrs male
•
•
•
•
•
Post renal transplant
cholecystitis cholecystectomy necrotising pancreatitis
Multiple Abs – CTX/MTZ/AMIK – MEM/AMIK/CPM
Mixed isolates from lap inc B. distasonis (MIC MTZ >32 mg/L, MEM >32 mg/L)
Pt died
Reports of clinical failure
associated with resistance
• Penicillin vs C. perfringens
– NIL
• Metronidazole vs Bacteroides spp
– YES
• Penicillin vs Bacteroides spp.
– YES
Brook (1984) Arch Otolaryngol 110: 228
Gudiol (1990) Arch Intern Med 150: 2525
Reports of clinical failure
associated with resistance
• Penicillin vs C. perfringens
– NIL
• Metronidazole vs Bacteroides spp
– YES
• Penicillin vs Bacteroides spp.
– YES
• Β-lactam/β-lactamase inhibitors vs Bacteroides
spp.
– NIL
• Carbapenems vs Bacteroides spp.
– YES
• 38 year old female
–
–
–
–
–
–
–
Elective laparotomy for adhesions
Post-op IA collection treated with co-amox
Day 13 - surgical drainage & change to CTX + MTZ
BC grew B. fragilis (isolate 1)
Changed to imipenem
2 weeks later persistent empyema drained (isolate 2)
Cured with drainage/clindamycin/gentamicin
Turner et al (1995) Lancet 345: 1275
• Prospective
evaluation of 128
patients with
Bacteroides
bacteraemia
CID (2000) 30: 870
Any data relating level of
resistance to outcome?
• NO
Conclusions
• Antimicrobial resistance is variably predictable
• Resistance rates are increasing
– CLD – becoming common
– MTZ + carbapenems – emerging
• Inducibility is a concern
• Correlation between in vitro resistance and
outcome has not been established for many
anaerobic infections
– The role of surgery should not be forgotten
Finegold 1989
• Susceptibility testing of anaerobes should be done
in 4 settings:
–
–
–
–
Determine patterns of susceptibility to new agents
Monitor susceptibility patterns Nationally
Monitor susceptibility patterns locally
Assist in the management of individual patients
• Persistence of infection/ failure of usual regimes/ difficulty
making decisions based on precedent
• Brain abscess/ endocarditis/ osteomyelitis/ prosthetic device
infection/ septic arthritis