Common infections• Sinusitus• 30-40% Streptococcus pneumoniae (S. pneumo)

• 20% Haemophilus influenzae (H. flu)• 20% Moraxella catarrhalis (M. cat)

• Acute otitis media (middle ear infection)• By age 3, up to 70% of children have experienced at least 1 infection• 15-25% M. catarrhalis; 90-95% of these isolates produce beta-lactamases• 40% S. pneumoniae; 30% penicillin-resistant (PRSP)• 30% H. influenzae; 30% produce beta-lactamases

• Community-acquired pneumonia• 50-80% S. pneumonia• 2-18% H. influenzae• 2-5% S. aureus• ~5% anaerobes• ~5% other Gram-

• By comparison: Hospital-acquired pneumonia• Pseudomonas aeruginosa• Staph. aureus• Klebsiella pneumoniae• Enterobacteriaceae (Enterobacter, E. coli, Proteus, Serratia marcescens, A. baumanii)

•Streptococcus pneumoniae: Gram+• Pneumonia (500,000 cases/yr); S. pneumo the most common cause• ~40,000 cases/yr of invasive pneumococcal infection• 28% resistant to at least one antibiotic• 11% resistant to 3 or more antibiotics

• Sinusitis and otitis media (7,000,000 cases/yr)• Sepsis (55,000 cases/yr)•Meningitis (6,000 cases/yr)• Penicillins front-line drugs, but now 30% is• penicillin resistant (PRSP), also multi-drug• resistance is observed• Vaccine helping to reduce prevalence• of antibiotic resistance

Respiratory, sinus and ear infections: S. pneumoniae

•Haemophilus influenzae: Gram-, aerobe/facultative anaerobe•Opportunistic commensal bacteria• Pneumonia• Sinusitis•Otitis media (middle ear infection)• Vaccine (HiB conjugate vaccine) is available• and has reduced frequency of invasive infections• relating to encapsulated serotype B• 30% beta-lactamase producing• Some show modified PBPs conferring• penicillin resistance, but ceph. may• be effective, macrolides, fluoroquinolones

Respiratory, sinus and ear infections: H. influenzae

Respiratory, sinus and ear infections: M. catarrhalis• Moraxella catarrhalis: Gram-, aerobic, commensal found in upper resp. tract

• Carriage in children up to 75%, in adults only 1-3%, esp in fall and winter• Emerged as pathogen, esp for children, adults w/ COPD, immune compromised• Otitis media (middle ear infection) • Pneumonia• Bronchitis• Sinusitis• Meningitis, sepsis more rare• Lower respiratory tract infections

• COPD patients• Pneumonia in elderly• Hospital outbreaks

Other common, big bugs๏Strep. pyogenes: Gram+; “group A streptococcus” (GAS), beta-hemolytic๏ Sometimes part of flora, nonpathogenic, asymptomatic๏ Skin and wound infections๏ 10,000,000 cases/yr infection: impetigo, cellulitis๏ 4500 cases/yr invasive infection: necrotizing fasciitis

๏ Strep throat๏ Scarlet fever๏ Streptococcal toxic shock๏Acute rheumatic fever; autoimmune reaction

triggered by Strep. pyogenes

✴Penicillin is the drug of choice, very littleresistance has emerged; for those penicillinallergic, macrolides, clindamycin

MRSA

๏ Methicillin-resistant (or multi-drug resistant) Staphylococcal aureus๏ penicillin, amoxacillin, oxacillin, cephalosporin, carbapenem resistant

๏ In hospitals, 50% of Staph. aureus is MRSA๏ ~90,000 infections and ~19,000 deaths/year in the US, more than from AIDS๏ ~30% of people naturally host Staph. aureus, ~2-3% MRSA

Janice H. Carr, C.D.C

MRSA๏ Hospital-associated MRSA (HA-MRSA) vs Community-associated MRSA (CA-MRSA),

two genetically distinct variants:๏ HA-MRSA (~86% of MRSA cases)๏ Resistant to: beta-lactams, macrolides, aminoglycosides, quinolones, clindamycin๏ Not as readily spread person-to-person๏ Recent trends suggest a decline in prevalence (30-50% from 2005 to 2008)

๏ CA-MRSA: (~14% of MRSA cases); probably a genetic hybrid of HA-MRSA and other๏ Resistant to beta-lactams, macrolides (somewhat more sensitive to treatment)๏ More readily spread person-to-person๏ PVL toxin more common; can cause tissue damage and recurrent disease/infection

๏ Virulence factors (depends on strain):๏ Exotoxins: e.g. Panton-Valentine Leukocidin (PVL) cytotoxin; necrotic lesions; PVL

gene carried by a bacteriophage hosted by S. aureus๏ Protein A: binds host immune proteins to its surface, stealth๏ Toxin superantigens: lipotheichoic acid (cytokine storm)๏ Enterotoxins๏ Alpha-hemolysin toxin induces apoptosis๏ Alters collagen tissues

Nosocomial infections and pathogens๏ ~10% infection rate (btxn 48h of admission to 30d after discharge); ~90,000 deaths/yr๏ Tend to exhibit greater frequencies and complexities of antimicrobial resistance๏ Often related to insertion devicesTypes of infections:

๏ Urinary tract infections (30%)๏ Hospital-aquired pneumonia and respiratory infections (22%)๏ Surgery-related (14%)๏ Skin and mucosa (7%)๏ Bacteremia 6%

Pathogens:๏ Pseuodomonas aeruginosa๏ Klebsiella pneumoniae๏ Staph. aureus๏ Enterococcus faecalis๏ Enterococcus faecium๏ Serratia marcescens๏ Proteus mirabilis๏ Enterobacter๏ Citrobacter๏ Stenotrophomonas maltophila๏ Acinetobacter baumanii๏ Burkholderia cepacia

๏Pseudomonas aeruginosa: Gram-, aerobic/facultative anaerobe, opportunist๏Minimal nutrient requirements, capable of colonizing wide range of environments๏ Frequent colonizer of medical device surfaces, e.g. catheters, ventilators

๏ Burn and wound infections๏ UTI๏ Gastrointestinal๏ Bone and joint๏ Bacteremia (blood infection)๏ Respiratory infections, cystic fibrosis๏ 10% of hospital-acquired infections๏ Significant antimicrobial resistance๏ Biofilm formation๏ Low cellular permeability to antibiotics๏ Selective porin channels and loss of expression of some porins๏ Porin deletion leads to lack of imipenem penetration,

๏ Overexpress outer-membrane protein (oprH) that stabilize lipopolysaccharide, which aminoglycoside usually interacts with to penetrate cell

๏ Efflux pumps; multi-drug efflux pumps transport across BOTH membranes๏ MexAB: beta-lactams, fluoroquinolones๏ MexXY: aminoglycosides๏ MexEF: carbapenems, fluoroquinolones

Nosocomial pathogens: example P. aeruginosa

๏ P. aeruginosa significant antimicrobial resistance (cont.)๏ Beta-lactamase expression๏ AmpC “cephalosporinase” (not susceptible to inhibitors)๏ Extended spectrum beta-lactamases (cephs, pens), sensitive to inhibitors๏ Carbapenemases

๏ Modified targets๏ Mutated DNA gyrase to decrease fluoroquinolone activity๏ Altered ribosomal targets for streptomycin but anti-pseudomonal aminoglycosides

are available๏ These resistance mechanisms may be observed in various combinations, strain and

isolate dependent

within the cytoplasm. The modifying enzymes are plasmid-mediated, consequently spontaneous mutations in cellsduring antibiotic treatment does not lead to over-expression of the enzymes, as seen with the chromosomalb-lactamases. Acquisition of the genes for the modifyingenzymes would require transfer from strains bearing theplasmids. Currently treatment of P. aeruginosa infections inCF with aerosolized tobramycin does not appear to haveresulted in increased resistance rates18.

Changes in targets

This mechanism of resistance results from mutationalchanges in target enzymes which result in maintenance oftheir vital role in cell metabolism but resistance to theaction of selective inhibition by antibiotics. In P. aeruginosa itis most commonly encountered with the quinolonesthrough mutation in the gyrA gene encoding the A subunitof the target enzyme, DNA gyrase19. Together with activeefflux this accounts for the current level of resistance seenin CF strains. Changes in the structure of the ribosome 30Ssubunit (the aminoglycoside target) influence streptomycinsensitivity but not that of the anti-pseudomonal aminoglyco-sides. Alteration in the penicillin-binding proteins ofP. aeruginosa resulting in resistance to b-lactams has beenreported but is not currently a major problem in CFstrains20.

Biofilms and resistance

In CF lung infections P. aeruginosa grows as aggregates ofcells (microcolonies) encased in a protective alginatepolysaccharide. This mode of growth also occurs onsurfaces, where it is referred to as a biofilm. Thecharacteristic property of all biofilms is their remarkableresistance to eradication by physical and biochemicaltreatments, including antibiotics21. Although this recalci-trance has been recognized for many years its biologicalbasis has still not been thoroughly explained. Factors whichmight partly explain the resistance phenotype include thehigh bacterial cell density and physical exclusion of theantibiotic. Physiological changes might occur in cells withinthe biofilm involving a general stress response, in which keymetabolic processes are shut down and protectivemechanisms induced22. It is clear that cells in the biofilm,like free-living ‘planktonic’ cells, can sense the presence ofother cells (quorum sensing) and alter their propertiesaccordingly23.

Finally, the population of cells within a biofilm isheterogeneous, containing fast- and slow-growing cells,some resistant through expression of inactivating enzymesand efflux pumps, others conspicuously not expressing suchsystems. The overall resistance is therefore dependent uponan interaction between the entire population of cells and

therapy needs to be directed against a multicellularcommunity21.

CONCLUSIONS AND IMPLICATIONS FORTHERAPY

Table 2 summarizes the contribution of different mechan-isms to the current resistance levels of P. aeruginosaencountered in CF. Restricted permeability and effluxare common components of the resistance phenotype forb-lactams, aminoglycosides and quinolones and areessentially fundamental properties of the organism. Theinnate antibiotic resistance of P. aeruginosa results from therestricted permeability of the cell wall and is enhanced bythe activity of efflux systems.

The occurrence of more specific mechanisms involvinginactivation and changes in targets reflects the selectivepressure resulting from heavy reliance on these agents inCF. For example, spontaneous mutations can increase theexpression of chromosomal b-lactamase genes. Thesemutants will be selected under the pressure of antibioticusage, especially where monotherapy is employed.

The increased recognition of the role of efflux systems ingeneral antibiotic resistance has led to a search for effluxpump inhibitors as therapeutic adjuncts.

Similarly, an understanding of the complex interactionin biofilm communities may eventually lead to novelstrategies for their control. However, P. aeruginosa hasalways proven to possess an answer to antibiotic therapy,the ominous size of its genome and current lack ofknowledge of the function of many of its genes suggests thatthis will continue to be the case whatever new therapies aredevised.

REFERENCES

1 Hancock RE. Resistance mechanisms in Pseudomonas aeruginosa and othernonfermentative gram-negative bacteria. Clin Infect Dis 1998;1(suppl):S93–9 25

J O U R N A L O F T H E R O Y A L S O C I E T Y O F M E D I C I N E S u p p l e m e n t N o . 4 1 V o l u m e 9 5 2 0 0 2

Table 2 Summary of resistance mechanisms of P. aeruginosa in cysticfibrosis

Antibiotic

Permeability

and efflux Inactivation

Changes in

targets

b-lactams +++ +++ +

Aminoglycosides ++ ++ 7

Quinolones +++ 7 +++

Polymyxins 7 7 +

+++ most commonly encountered; ++ common; + reported but rare; 7 not reported

Lambert PA, J. Royal Soc. Med., (2002) 95(suppl.41):22

Nosocomial pathogens: example P. aeruginosa