Clinical data to support the interpretation of susceptibility testing of anaerobes

1
Clinical data to support the interpretation of
susceptibility testing of anaerobes

• Robin Howe

2
'Mixed anaerobes sensitive to metronidazole' (comm
on practice in UK)
Pus from cerebral abscess Primary plate after 5
days AnO2 incubation
Courtesy Val Hall
3
? 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
4
'S.O.P.' for susceptibility testingof 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
5
Why is susceptibility testing of anaerobes not
generally used in clinical decision-making?

• 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)

6
Are resistance rates predictable?Does in vitro
resistance correlate with an identifiable
resistance mechanism?
Hecht (2004) CID39 92
7
Are resistance rates predictable?Does in vitro
resistance correlate with an identifiable
resistance mechanism?
Hecht (2004) CID39 92
8

• Baquero (1992)
• 10 C. perfringens in Spain resistant to pen
  (MIC gt0.5 mg/L)

BSAC BP 0.12 mg/L
9
Are resistance rates predictable?Does in vitro
resistance correlate with an identifiable
resistance mechanism?

• B. fragilis
• Class 2e cephalosporinase
• V. common
• Inhibited by clavulanate etc
• Cephamycinases
• Uncommon
• cepA, cfxA
• Zinc metallo-?-lactamase
• cfiA, ccrA
• Present in 4 - not usually expressed

• Altered PBPs
• Rare
• Porin loss
• Reported

10
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. n78
• (5 of all Bacteroides)
• Clostridium paraputrificum
• n5 (4)
• Clostridium ramosum
• n3 (1)

11
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
12

• nim genes identified in 2 of 1,502 B. fragilis
  from 19 European countries

13

• nim genes identified in 2 of 1,502 B. fragilis
  from 19 European countries

14
Animal studies
The role of anti-anaerobic therapy

• 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 101256
15
Animal studies
16
?synergistic infection

• IP injection of mixtures of three orgs

Brook (1994) JAC 34 791
17
(No Transcript)
18
Reports of clinical failure associated with
resistance

• Penicillin vs C. perfringens
• NIL
• Metronidazole vs Bacteroides spp
• YES

19
Rotimi et al (1999)CMI 5 166

• 3 case reports
• 75 yrs female
• post op Hartmanns treated with CAZ/MTZ
• Readmitted with paracolic abscess
• B. frag isolated (MTZ MIC gt32 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 gt32mg/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 gt32 mg/L, MEM gt32 mg/L)
• Pt died

20
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
21
Reports of clinical failure associated with
resistance

• Penicillin vs C. perfringens
• NIL
• Metronidazole vs Bacteroides spp
• YES
• Penicillin vs Bacteroides spp.
• YES
• ?-lactam/ß-lactamase inhibitors vs Bacteroides
  spp.
• NIL
• Carbapenems vs Bacteroides spp.
• YES

22

• 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
23

• Prospective evaluation of 128 patients with
  Bacteroides bacteraemia

CID (2000) 30 870
24
(No Transcript)
25
Any data relating level of resistance to outcome?

• NO

26
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

27
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