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Resistance to b-lactam
Antibiotics
revisited
Dr.T.V.Rao MD
Dr.T.V.Rao MD 1
β-lactam Antibiotics
• The β-lactam ring is part of the
core structure of several
antibiotic families, the principal
ones ...
History of β-lactams
• The first synthetic β-lactam was
prepared by Hermann Staudinger
in 1907 by reaction of the Schiff
b...
Survival of the fittest
Dr.T.V.Rao MD
4
• Resistant bacteria survive, susceptible ones die
Mutant emerges
slowly
Sensitive...
Action of a b-lactamase
N
O
COOH
S
HN
O
COOH
S
OH
Active penicillin
Inactive penicilloateH2O
Mechanism of b-Lactam Action
••Bactericidal
•b-lactams bind and inhibit
penicillin binding proteins
(PBPs)
•PBPs are respo...
Spread of TEM plasmid
b-lactamases
• 1963 Ampicillin; 1st broad spectrum penicillin
• 1965 TEM b-lactamases in E. coli
• 1...
PPID, 6th ed. 2005
ESBL Introduction
• B-lactamases conferring resistance to the penicillin's, first-
second-, and third-g...
Mechanisms of GNR Resistance to
b-lactams
Porin-mediated resistance
• Antibiotic does not reach target
b-lactamase
• Major...
Dr.T.V.Rao MD 10
BETA LACTAM RING
PENICILLIN
BETA LACTAM RING
CEPHALOSPORIN
BETA LACTAMASES enzymes that
inactivate the be...
How are b-lactamases transferred?
• Transfer of Plasmids.
• Extra chromosomal DNA
• Usually carry antibiotic
resistance ge...
b-lactam antibiotics
• Penicillins
• Ampicillin
• Piperacillin
• Beta-lactam/beta-lactamase
inhibitors
• Ampicillin/sulbac...
The β-lactam family of antibiotics
Ceftriaxone 3rdTicarcillin
Ceftazidime 3rdMezlocillin
Cefotaxime 3rdCarbenicillin
Ertap...
b-lactam antibiotics
• First Generation
cephalosporins
• Cefazolin
• Cephalothin
• Second Generation oral
antibiotics
• Ce...
b-lactam antibiotics
• Third generation
cephalosporins
• Cefotaxime
• Ceftriaxone
• Ceftazidime
• Fourth generation
cephal...
b-lactam antibiotics
• Carbapenems
•Impenem
•Meropenem
•Ertapenem
•Doripenem
17
Plasmid-mediated TEM and SHV b-lactamases
Ampicillin
1965
TEM-1
E.coli
S.paratyphi
1970s
TEM-1
Reported in
28 Gm(-) sp
...
Classification of β lactamases
Dr.T.V.Rao MD 18
• Richards and Sykes (1971)
• substrate
• Ambler (1969)
• structure
• Bush...
Classification
Dr.T.V.Rao MD 19
•Ambler Classification
•Molecular class A – D
•A
•Bush-Jacoby-Medeiros Classification
•Fun...
ESBLs
• Enzymes capable of
hydrolyzing third-generation
cephalosporins.
• Plasmid-mediated
• Derivatives (mutants) of
orig...
Clin Microbiol Rev. 2005;18:657-686 J Clin
Microbiol.2001;39:2206-2212.
ESBL In Vitro Susceptibility
• NCCLs established b...
Clin Microbiol Infect. 2008;14:169-174.
ESBL In Vitro Susceptibility
• Currently accepted that cephalosporin breakpoints u...
E. coli susceptibility Report
• Ampicillin R
• Piperacillin R
• Cephalothin R
• Cefoxitin S
• Cefotaxime R
• Ceftazidime I...
Laboratory detection of ESBLs
•Resistance or intermediate to third-generation
cephalosporins.
•Cefoxitin and cefotetan sus...
Double disc antagonism for inducible AmpC
Cefoxitin Ceftazidime
ESBL Confirmatory Tests
Double-disk synergy (DDS) test
• CAZ and CAZ/CA disks
• CTX and CTXCA disks
• Confirmatory testing...
Combination Disk Method
CLSI Approved Method
28AmpC Disk Test
Lawn culture:
E. coli ATCC 25922
Test Organism
on disk
E. coli ESBL susceptibility
report
• Ampicillin R
• PiperacillinR
• Cephalothin R
• Cefoxitin S
• Cefotaxime R
• Ceftazidi...
Enterobacter cloacae susceptibility
report
• Ampicillin R
• PiperacillinR
• Cephalothin R
• Cefoxitin R
• Cefotaxime R
• C...
AmpC b-lactamases
•Chromosomally encoded-cell wall turnover
•Enterobacter sp., Citrobacter sp., Serratia
sp., Morganella s...
Other concepts to know about AmpC
b-lactamases
•They are transferred on plasmids as well.
•CMY, LAT, BIL, MOX, ACC, FOX, D...
Beta-lactamase inhibitors
Dr.T.V.Rao MD
•Resemble β-lactam antibiotic structure
•Bind to β-lactamase and protect the antib...
Resistance and genetics
AmpC
Hi-level
TEM
ESBL
CTX-M K1
Ceftazidime R R v S
Cefotaxime R v R S
Cefoxitin R S S S
Aztreonam...
Why Test for β-lactamases ?
 Improve clinical outcome
 Inappropriate treatment leads to poor outcome
 Each 1 hour delay...
Clin Microbiol Rev. 2005;18:657-686.
ESBL Epidemiology
• North America
• National Nosocomial Infections Surveillance (NNIS...
Mechanisms of Carbapenem
Resistance
•Carbapenemase
hydrolyzing
enzymes
•Porin loss “OprD”
•ESBL or AmpC +
porin loss
Carbapenemases
•The most versatile family of b-lactamases
•Two major groups based on the hydrolytic mechanism
at the activ...
Carbapenemase Classification
Molecular
Class
A B D
Functional
Group
2f 3 2d
Aztreonam
Hydrolysis
+ - -
EDTA
Inhibition
- +...
Klebsiella pneumoniae
• Ampicillin R
• PiperacillinR
• Cephalothin R
• Cefoxitin S
• Cefotaxime R
• Ceftazidime I
• Ceftri...
Carbapenemases Class A
• First identified 1982 in UK
• Four major families
• Chromosomally encoded
• Serratia marcescens e...
Etest for metallo-b-lactamase
Imipenem
Imipenem
+
EDTA
Etest for metallo-b-lactamase
Imipenem
Imipenem
+
EDTA
KPC
•Molecular class A and functional group 2f
•Inhibited by clavulanic acid but not by EDTA
•Confers resistance to ALL b-...
KPC Epidemiology
• Predominantly in K. pneumoniae
(KP)
• Reported in Enterobacter spp.,
Salmonella spp., E. coli, P.
aerug...
When to Suspect a KPC Producer
• Enterobacteriaceae
• Resistance to extended
spectrum cephalosporins
(cefotaxime, ceftazid...
How to Detect a KPC Producer
• Antimicrobial susceptibility tests
(ASTs)
• MIC
• Carbapenem MIC  2
g/ml
• Disk diffusion...
How to Detect a KPC Producer
• Commercial systems
•Inconsistent detection
of KPC-producing
isolates
• Tenover et al.
2006....
Definitive ID of a KPC
Producer
• Modified Hodge test
• 100% sensitivity to detect KPC
1. Swab E. coli ATCC 25922 onto
pla...
Definitive ID of a KPC Producer
• PCR
•The method
of choice to
confirm KPC
Alternative Treatment for a
KPC Producer
• Tigecycline (100.0% effective)
• Colistin (88.1% effective)
• SENTRY report. AA...
Clin Microbiol Rev. 2005;18:657-686.
ESBL Antibiotic Choice
• Cefepime should not be used as first-line against ESBL-produ...
Conclusions
•ESBL detection—CLSI guidelines present
•Need to have guidelines to detect ESBLs present in
other species besi...
The message
• Beta-lactamases are getting more complex
• Full I/D needs complex molecular methods
Much can be inferred fro...
Hand washing still can reduce the ESBL
spread in the Hospitals
Dr.T.V.Rao MD 55
Dr.T.V.Rao MD 56
• The Programme created by Dr.T.V.Rao MD for
Medical Microbiologists in the Developing
World
•Email
•doct...
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Resistance to b lactam antibiotics

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Transcript of "Resistance to b lactam antibiotics"

  1. 1. Resistance to b-lactam Antibiotics revisited Dr.T.V.Rao MD Dr.T.V.Rao MD 1
  2. 2. β-lactam Antibiotics • The β-lactam ring is part of the core structure of several antibiotic families, the principal ones being the penicillins, cephalosporins, carba penems, and monobactams, which are, therefore, also called β- lactam antibiotics. Nearly all of these antibiotics work by inhibiting bacterial cell wall biosynthesis. This has a lethal effect on bacteria
  3. 3. History of β-lactams • The first synthetic β-lactam was prepared by Hermann Staudinger in 1907 by reaction of the Schiff base of aniline and benzaldehyde with diphenylketene in a cycloaddition: • Upto 1970, most β-lactam research was concerned with the penicillin and cephalosporin groups, but since then a wide variety of structures have been described.
  4. 4. Survival of the fittest Dr.T.V.Rao MD 4 • Resistant bacteria survive, susceptible ones die Mutant emerges slowly Sensitive cells killed by antibiotic Mutant’s progeny overrun
  5. 5. Action of a b-lactamase N O COOH S HN O COOH S OH Active penicillin Inactive penicilloateH2O
  6. 6. Mechanism of b-Lactam Action ••Bactericidal •b-lactams bind and inhibit penicillin binding proteins (PBPs) •PBPs are responsible for assembly, maintenance, and regulation of peptidoglycan (cell wall) metabolism. •Disruption of peptidoglycan synthesis
  7. 7. Spread of TEM plasmid b-lactamases • 1963 Ampicillin; 1st broad spectrum penicillin • 1965 TEM b-lactamases in E. coli • 1969 TEM b -lactamase in P. aeruginosa • 1974 TEM in H. influenzae & N. gonorrhoeae • Now TEM in 30-60% E. coli & enterobacteria & in 5- 20% of H. influenzae & gonococci
  8. 8. PPID, 6th ed. 2005 ESBL Introduction • B-lactamases conferring resistance to the penicillin's, first- second-, and third-generation cephalosporins and aztreonam • Mechanism is via hydrolysis • Inhibited by B-lactamase inhibitors such as clavulanic acid • B-lactamases in group 2d and group 2be • Group 2b: TEM-1, TEM-2, & SHV-1 • Group 2d: OXA • B-lactamase in group 1 • AmpC*
  9. 9. Mechanisms of GNR Resistance to b-lactams Porin-mediated resistance • Antibiotic does not reach target b-lactamase • Majority of resistance to b- lactam antibiotics mediated through b-lactamases. • Many different types of b- lactamases with different substrate (antibiotic) specificities.
  10. 10. Dr.T.V.Rao MD 10 BETA LACTAM RING PENICILLIN BETA LACTAM RING CEPHALOSPORIN BETA LACTAMASES enzymes that inactivate the beta-lactam ring
  11. 11. How are b-lactamases transferred? • Transfer of Plasmids. • Extra chromosomal DNA • Usually carry antibiotic resistance genes • These genes can be encoded on transposons, which are also mobile. • TEM-1 has been transferred between the Enterobacteriaceae and H. influenzae and the Neisseriaceae
  12. 12. b-lactam antibiotics • Penicillins • Ampicillin • Piperacillin • Beta-lactam/beta-lactamase inhibitors • Ampicillin/sulbactam • Amoxicillin/clavulanate • Ticarcillin/clavulanate • Piperacillin/Tazobactam
  13. 13. The β-lactam family of antibiotics Ceftriaxone 3rdTicarcillin Ceftazidime 3rdMezlocillin Cefotaxime 3rdCarbenicillin ErtapenemCefmetazoleCefuroxime 2ndAmpicillin MeropenemCefotetanCefamandole 2ndMethicillin AztreonamImipenemCefoxitinCephalothin 1stBenzyl- penicillin MonobactamsCarbapenemsCephamycinsCephalosporinsPenicillins Cefepime 4th
  14. 14. b-lactam antibiotics • First Generation cephalosporins • Cefazolin • Cephalothin • Second Generation oral antibiotics • Cefuroxime (many others) • Second Generation cephamycins • Cefoxitin
  15. 15. b-lactam antibiotics • Third generation cephalosporins • Cefotaxime • Ceftriaxone • Ceftazidime • Fourth generation cephalosporins • Cefepime • Monobactams • Aztreonam
  16. 16. b-lactam antibiotics • Carbapenems •Impenem •Meropenem •Ertapenem •Doripenem
  17. 17. 17 Plasmid-mediated TEM and SHV b-lactamases Ampicillin 1965 TEM-1 E.coli S.paratyphi 1970s TEM-1 Reported in 28 Gm(-) sp 1983 ESBL in Europe 1988 ESBL in USA 2000 > 130 ESBLs Worldwide Extended-spectrum Cephalosporins 1963 Evolution of b-Lactamases Look and you will find ESBL
  18. 18. Classification of β lactamases Dr.T.V.Rao MD 18 • Richards and Sykes (1971) • substrate • Ambler (1969) • structure • Bush, Jacoby, Medeiros (1995) • Substrate; correlation with molecular structure • 150 TEM; • 88 SHV; • 88 OXA, • 53 CTX-M; • 22 IMP; • 12 VIM + smaller number of other enzymes (http://www.lahey.o
  19. 19. Classification Dr.T.V.Rao MD 19 •Ambler Classification •Molecular class A – D •A •Bush-Jacoby-Medeiros Classification •Functional group 1 – 4 •2 •2b •2be Paterson and Bonomo, 2005
  20. 20. ESBLs • Enzymes capable of hydrolyzing third-generation cephalosporins. • Plasmid-mediated • Derivatives (mutants) of original TEM-1 and SHV-1 b- lactamases. • Susceptible in-vitro to clavulanate and cefoxitin.
  21. 21. Clin Microbiol Rev. 2005;18:657-686 J Clin Microbiol.2001;39:2206-2212. ESBL In Vitro Susceptibility • NCCLs established breakpoints 1980s • In vitro, MICs of ceph rise as inoculum of ESBL prod organisms rise “inoculum effect” • NCCLs subcommittee convened working group recommending • K. spp and E. coli screened for ESBL prod • Suspected ESBL tested for phenotypic confirmation • 1998 survey of 369 laboratories only 32% performed tests to detect ESBL production • Most liberal interpretation of ceph susceptibility by CLSI w/ MIC</=8ug/ml
  22. 22. Clin Microbiol Infect. 2008;14:169-174. ESBL In Vitro Susceptibility • Currently accepted that cephalosporin breakpoints used in Europe (EUCAST) and US (CLSI) fail to detect most ESBL • Published data suggests that clinical outcome with 3rd gen ceph related more to MICs and not presence of ESBL arguing against “inoculum effect” • New breakpoints adopted by EUCAST March 2006 • Existing breakpoints do not allow for detection of important resistance mechanisms • Question if breakpoints correlate with clinical outcome • Controversy re: contradicting 3rd gen ceph as S or R is ESBL pos • CLSI Working Group on Enterobacteriacea have been proposed but not accepted as of Jan 2008 • Suggested CLSI breakpoints for senstivity pre/post (ug/ml) • Cefuroxime (8/8), Cefotaxime (8/1 ), Ceftriaxone (8/1), Ceftazidime (8/4), Cefepime (8/8)
  23. 23. E. coli susceptibility Report • Ampicillin R • Piperacillin R • Cephalothin R • Cefoxitin S • Cefotaxime R • Ceftazidime I • Ceftriaxone R • Aztreonam I • Cefepime S • Pip/Tazo I • Imipenem S
  24. 24. Laboratory detection of ESBLs •Resistance or intermediate to third-generation cephalosporins. •Cefoxitin and cefotetan susceptible. •ESBL disk diffusion test (clavulanate inhibition) •E-test ESBL strip •Confirmatory ESBL MIC test (Microscan) •K. pneumoniae, K. oxytoca, E. coli, P. mirabilis
  25. 25. Double disc antagonism for inducible AmpC Cefoxitin Ceftazidime
  26. 26. ESBL Confirmatory Tests Double-disk synergy (DDS) test • CAZ and CAZ/CA disks • CTX and CTXCA disks • Confirmatory testing requires using both CAZ and CTX alone and with CA • 5 mm enhancement of the inhibition zone of antibiotic/CA combination vs antibiotic tested alone = ESBL
  27. 27. Combination Disk Method CLSI Approved Method
  28. 28. 28AmpC Disk Test Lawn culture: E. coli ATCC 25922 Test Organism on disk
  29. 29. E. coli ESBL susceptibility report • Ampicillin R • PiperacillinR • Cephalothin R • Cefoxitin S • Cefotaxime R • Ceftazidime IR • Ceftriaxone R • AztreonamIR • Cefepime SR • Pip/Tazo I • Imipenem S
  30. 30. Enterobacter cloacae susceptibility report • Ampicillin R • PiperacillinR • Cephalothin R • Cefoxitin R • Cefotaxime R • Ceftazidime I • Ceftriaxone R • AztreonamI • Cefepime S • Pip/Tazo R • Imipenem S
  31. 31. AmpC b-lactamases •Chromosomally encoded-cell wall turnover •Enterobacter sp., Citrobacter sp., Serratia sp., Morganella sp. Even E. coli. •Third-generation cephalosporins are not good inducers of AmpC b-lactamase •Third-generation cephalosporin resistant strains are derepressed—meaning that the AmpC b- lactamase is not inducible anymore. •AmpC mutants are cephamycin resistant
  32. 32. Other concepts to know about AmpC b-lactamases •They are transferred on plasmids as well. •CMY, LAT, BIL, MOX, ACC, FOX, DHA •Almost all ceftriaxone-resistant Salmonella isolated in the United States carry a plasmid-mediated AmpC b-lactamase called CMY-2. •E. coli UTI isolates carry plasmid-mediated
  33. 33. Beta-lactamase inhibitors Dr.T.V.Rao MD •Resemble β-lactam antibiotic structure •Bind to β-lactamase and protect the antibiotic from destruction •Most successful when they bind the β-lactamase irreversibly •Three important in medicine •Clavulanic acid •Sulbactam •Tazobactam
  34. 34. Resistance and genetics AmpC Hi-level TEM ESBL CTX-M K1 Ceftazidime R R v S Cefotaxime R v R S Cefoxitin R S S S Aztreonam R v v R Synergy + clav No +++ +++ No Dr.T.V.Rao MD 34 Know the species
  35. 35. Why Test for β-lactamases ?  Improve clinical outcome  Inappropriate treatment leads to poor outcome  Each 1 hour delay increases mortality by 7.6% in septic shock1  Encourage antimicrobial stewardship  Spare carbapenems..  Reduce C. difficile / antibiotic associated diarhoea  Enhanced surveillance  Identify emerging resistance problems  Develop structures to prevent dissemination  Infection Control  ‘Search and Destroy’ analogous to MRSA ?  Laboratory Detection is not always easy… OR Rapid 1Kumar, Crit Care Med, 2006
  36. 36. Clin Microbiol Rev. 2005;18:657-686. ESBL Epidemiology • North America • National Nosocomial Infections Surveillance (NNIS) Jan 1998-June 2002 • 6.1% of Klebsiella pneumoniae isolates resistant to 3rd gen ceph in 110 ICUs • >10% of ICUs, resistance exceeds 25% • Non-ICU inpt, 5.7% of Klebsiella pneumoniae isolates resistant • Outpt, 1.8% of Klebsiella pneumoniae resistant • Prevalence of ESBL underestimated due to MIC S/I • Europe • France in early 1990s, 25-35% of nococomial Klebsiella pneumoniae were ESBL producing • N. France in 2000, 7.9% of nosocomial Klebsiella pneumoniae were ESBL producing • Discordance between Western and Eastern Europe
  37. 37. Mechanisms of Carbapenem Resistance •Carbapenemase hydrolyzing enzymes •Porin loss “OprD” •ESBL or AmpC + porin loss
  38. 38. Carbapenemases •The most versatile family of b-lactamases •Two major groups based on the hydrolytic mechanism at the active site •Serine at the active site: class A and D •Zinc at the active site: class B •All carbapenemases hydrolyze penicillins, extended spectrum cephalosporins, and carbapenems
  39. 39. Carbapenemase Classification Molecular Class A B D Functional Group 2f 3 2d Aztreonam Hydrolysis + - - EDTA Inhibition - + - Clavulanate Inhibition + - 
  40. 40. Klebsiella pneumoniae • Ampicillin R • PiperacillinR • Cephalothin R • Cefoxitin S • Cefotaxime R • Ceftazidime I • Ceftriaxone R • AztreonamI • Cefepime S • Pip/Tazo R • Imipenem I • Might need to screen for carbapenemase
  41. 41. Carbapenemases Class A • First identified 1982 in UK • Four major families • Chromosomally encoded • Serratia marcescens enzyme (SME) • Not metalloenzyme carbapenemases (NMC) • Imipenem-hydrolyzing b-lactamases (IMI) • Plasmid encoded • Klebsiella pneumoniae carabapenemases (KPC) • Guiana Extended-Spectrum (GES)
  42. 42. Etest for metallo-b-lactamase Imipenem Imipenem + EDTA
  43. 43. Etest for metallo-b-lactamase Imipenem Imipenem + EDTA
  44. 44. KPC •Molecular class A and functional group 2f •Inhibited by clavulanic acid but not by EDTA •Confers resistance to ALL b-LACTAM antibiotics •Plasmid-encoded •Associated with other resistant genes (aminoglycosides, fluoroquinolones) •Transferable
  45. 45. KPC Epidemiology • Predominantly in K. pneumoniae (KP) • Reported in Enterobacter spp., Salmonella spp., E. coli, P. aeruginosa, and Citrobacter spp. • First identified in KP clinical isolate from North Carolina in 1996 (KPC- 1) • KPC-2, -3, and -4 have been reported. • Mostly identified on the East cost
  46. 46. When to Suspect a KPC Producer • Enterobacteriaceae • Resistance to extended spectrum cephalosporins (cefotaxime, ceftazidime, and ceftriaxone) • Variable susceptibility to cephamycins (cefoxitin, cefotetan) • Carbapenem MICs  2 g/ml
  47. 47. How to Detect a KPC Producer • Antimicrobial susceptibility tests (ASTs) • MIC • Carbapenem MIC  2 g/ml • Disk diffusion • Carbapenem: “I” or “R” • Among carbapenems, ertapenem: • Most sensitive • less specific Anderson et al. 2007. JCM 45 (8): 2723
  48. 48. How to Detect a KPC Producer • Commercial systems •Inconsistent detection of KPC-producing isolates • Tenover et al. 2006. EID. 12:1209-1213 •Breakpoints do not match CLSI recommendations
  49. 49. Definitive ID of a KPC Producer • Modified Hodge test • 100% sensitivity to detect KPC 1. Swab E. coli ATCC 25922 onto plate to create lawn Place imipenem disk in center. 2. Streak test isolates from edge of disk to end of plate. 3. Incubate overnight. 4. Look for growth of E. coli around test isolate streak - indicates carbapenem-hydrolyzing enzyme. meropenem ertapenem imipenem pos neg neg neg pos pos Janet Hindler, What’s New in the 2008 CLSI Standards for (AST)?
  50. 50. Definitive ID of a KPC Producer • PCR •The method of choice to confirm KPC
  51. 51. Alternative Treatment for a KPC Producer • Tigecycline (100.0% effective) • Colistin (88.1% effective) • SENTRY report. AAC. 2008. Feb;52(2):570-3 • Minocycline • A strategy for susceptibility testing is needed
  52. 52. Clin Microbiol Rev. 2005;18:657-686. ESBL Antibiotic Choice • Cefepime should not be used as first-line against ESBL-producing organisms • MICs rise with inoculum effect size • High dose 2 gm iv 12 +/- amikacin • B-lactam/B-lactamase inhibitor • MICs rise with inoculum size • Reduced activity in presence of porin loss and b-lactamase production • Quinolones option for complicated UTI due to ESBL organism • In vitro synergy with fq + b-lactam (cefotax) • Carbapenems first line for serious ESBL organisms • Meropenem preferred over Imipenem for nosocomial meningitis • No evidence of combination superior to alone
  53. 53. Conclusions •ESBL detection—CLSI guidelines present •Need to have guidelines to detect ESBLs present in other species besides E. coli, K. pneumoniae, K. oxytoca, and P. mirabilis. •AmpC detection-No guidelines available •KPC detection-Not widespread, need to have lower concentrations of carbapenems on panels.
  54. 54. The message • Beta-lactamases are getting more complex • Full I/D needs complex molecular methods Much can be inferred from simple tests. Needs I/D Testing wide panels of antibiotics; synergy tests Knowledge of what’s unusual
  55. 55. Hand washing still can reduce the ESBL spread in the Hospitals Dr.T.V.Rao MD 55
  56. 56. Dr.T.V.Rao MD 56 • The Programme created by Dr.T.V.Rao MD for Medical Microbiologists in the Developing World •Email •doctortvrao@gmail.com
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