Circuit Newsletter 2023 Edition 3 - Slade Pharmacy

2023 Edition 3

CLINICAL INITIATIVES, RESEARCH AND CURRENT UPDATES IN TREATMENT

World AMR Awareness Week, 18 - 24 November: Preventing Antimicrobial Resistance Together

(Mis)understanding the micro: Antimicrobial susceptibility testing

Kirby-Bauer Disk Diffusion Antimicrobial Susceptibility Test

Terese Tinoo and Annika Turner, Epic Pharmacy Hollywood Antimicrobial susceptibility testing (AST) is a laboratory method used to determine the effectiveness of antimicrobials against a specific pathogen.1 These tests are commonly used to help guide healthcare providers in choosing the most appropriate antibiotic for treating a bacterial infection.

Antimicrobial Susceptibility Testing (AST) involves1 (see Figure 1): 1.

Isolation of Bacterial Strain: A sample is collected from the patient, and the bacteria causing the infection are isolated and grown on an agar plate in the laboratory.

2.

Inoculation of Antibiotic Discs: A standardized amount of the bacterial culture is spread evenly on the surface of an agar plate. Small discs containing different antibiotics are placed on the agar surface.

3.

Incubation: The agar plate is incubated under controlled conditions to allow the bacteria to grow.

4.

Zone of Inhibition: After incubation, the antibiotics diffuse from the discs into the surrounding agar. If the bacteria are susceptible to an antibiotic, it will not grow in the area around the antibiotic disc.1 This clear zone around the disc is called the “zone of inhibition.”

5.

Measurement of Zones: The diameter of the zone of inhibition is measured using a ruler or caliper. The size of the zone is directly related to the sensitivity of the bacteria to the antibiotic; larger zones indicate greater susceptibility.

6.

Interpretation: The zone sizes are compared to standardized interpretive criteria established by organisations like the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST) (see Figure 2).1,2

Step 1 Preparation of disk containing samples of antibiotics

Step 2 Preparation of test bacterium culture

Step 3 Place disk on the bacterium culture

Step 4 Measurement of the inhibition zones’ thickness

Figure 1: Representation of disk diffusion test for evaluation of antimicrobial susceptibility2

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Figure 2: Interpretations of susceptibility reporting definitions3

S I

“Susceptible” = zone of inhibition indicates a high likelihood of therapeutic success using standard dosing of antibiotic

“Intermediate” = traditional term to indicate a technical uncertainty buffer zone, where blood concentrations are possible but clinical response rates may be lower than those for susceptible isolates “Susceptible dose-dependent” = category additionally applied by CLSI, only used if there is a possibility of higher drug exposure through dosing “Susceptible, increased exposure” =used by EUCAST, indicating success due to increased exposure to the drug

R

“Resistant” =zone of inhibition is small or non-existent; high likelihood of therapeutic failure with increased exposure.

The traditional definition of ‘I’ represented an ‘Intermediate’ result and did not adequately guide clinical practice, and many clinicians may have regarded it as another ‘R’.4 It allowed opportunity for confusion and misinterpretation, for example whether or not the organism was susceptible to the antimicrobial reported, if the organism was susceptible depending on the site of infection, if a laboratory technical error had occurred or if the susceptibility result was inconclusive.4 The new modified definitions of the ‘I’ category clarified that the antibiotic remained susceptible, provided that dose augmentation from the standard dosage occurred.4 Exposure to an antibiotic can be increased by altering the mode of administration, increasing the

Example: Here is an example of these definitions when reporting on Pseudomonas aeruginosa (Figure 34).

Microbiology laboratories in Australia follow the Therapeutic Guidelines, aiming to report narrow spectrum antibiotics, at least one oral and IV agent and one agent for penicillin allergy. The treatment of infections with P. aeruginosa requires increased exposure for almost all relevant antimicrobials, except meropenem.4 Although meropenem is one of the only antimicrobials reported as ‘S’, this does not mean that it is the preferred option. P.aeruginosa is susceptible to all the antibiotics reported as ‘I’, however, they all have to be prescribed with a modified dosage regime to achieve efficacy.4 Refer to the following article (Mis)understanding the micro: Pseudomonas aeruginosa for dosing examples.

dosage, decreasing dosing intervals and prolonging infusion times.4 However, while many pathology laboratories have switched over to using the new “I” definitions, many are yet to change. Along with differences depending on whether the laboratory follows EUCAST or CLSI guidelines, there are currently inconsistencies with how “I” is reported, of which many clinicians may be unaware, leading to misinterpretation. If a patient has results from different laboratories from the community, public hospitals and private hospitals, they may all require a different interpretation of an “I” result. It is therefore important to read any explanation of definitions included in the report.

Figure 3: Specimen/Site Leucocytes Gram Negative Bacilli Culture: Pseudomonas aeruginosa Amoxicillin Amoxicillin-clavulanate Cefazolin Ciprofloxacin Cotrimoxazole Ceftazidime Cefepime Meropenem Piperacillin-tazobactam Tobramycin**

Subhepatic Abscess + +++ Heavy growth OLD R R R S R S S S S S

NEW R R R I R I I S I S

**used in combination with other active therapy

ALTHOUGH MEROPENEM IS ONE OF THE ONLY ANTIMICROBIALS REPORTED AS ‘S’, THIS DOES NOT MEAN THAT IT IS THE PREFERRED OPTION.

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Minimum Inhibitory Concentration (MIC) & Breakpoints Agar dilution involves the incorporation of an antibiotic in solid or semi-solid agar media in a geometrical progression of concentrations and the application of a volume of bacteria to the surface.5 Its purpose is to determine the lowest concentration that inhibits bacterial growth, called the minimum inhibitory concentration (MIC). 5 The responsible bacteria are susceptible or resistant if the antibiotic MIC is below or above the clinical breakpoint cut-off, respectively. This determines whether susceptibility is reported as S, I or R. The EUCAST and CLSI annually updates the clinical breakpoint tables for the interpretation of MICs.5

Antimicrobial Susceptibility Testing is a valuable tool in guiding the selection of appropriate antibiotics for treating bacterial infections. However, it does have some limitations that healthcare providers need to be aware of when interpreting the results and making treatment decisions.

1

Time-consuming process: Traditional AST methods can take 24 to 48 hours or even longer to yield results. Empirical broad-spectrum antibiotic therapy might be required.6

2

Emergence of antibiotic resistance: AST results are based on the current susceptibility of the bacterial strain to antibiotics. However, bacterial populations can evolve quickly and resistance can develop during the course of treatment.6

3

Interpretive criteria: The determination of interpretive criteria for classifying bacteria as sensitive, intermediate OR susceptible- increased exposure and resistant can vary among different organisations and regions.

Slight variations in the criteria can impact treatment decisions as the same result

could be reported differently between laboratories, and potentially erroneously interpreted.3,6

4

Limited spectrum: AST typically focuses on a specific set of antibiotics commonly used in clinical practice. It may not include newer or less commonly prescribed antibiotics.6,7

7

Inability to predict in vivo response: AST is performed in vitro and factors such as antibiotic penetration into tissues, host immune response, and drug interactions cannot be fully accounted for.6,7

8

Biofilm formation: Some bacteria can form biofilms, which are protective layers that make them more resistant to antibiotics.6,7

5

9

6

10

Inadequate sample collection: Improper sample collection or contamination during collection can lead to misleading results.6,7 Mixed infections: Infections can be caused by more than one bacterial strain or species, leading to mixed infections. AST may not accurately represent the susceptibility patterns of each strain.6,7

Host factors: The effectiveness of an antibiotic can be influenced by patient immune status, underlying health conditions and drug metabolism.6,7 Uncommon bacteria: Some bacteria are difficult to grow in the laboratory, making it challenging to perform AST for these organisms.6,7

The categories I and S must be analysed together with pharmacokinetic (PK) parameters that describe the passage of the drug in the body, including its absorption and metabolism. For example, the use of standard doses of antimicrobials in critically ill patients in ICU is often associated with sub-optimal exposure. ICU patients can be often described as having altered PK, such as increased volume of distribution and increased or augmented renal clearance related to increased renal blood flow due to septic shock, and therefore may require dose alterations to increase the exposure.6,7,8

Despite any limitations, AST remains an essential tool for guiding antibiotic therapy. It provides valuable information to healthcare providers, but it should be interpreted in conjunction with the patient’s clinical presentation.6,7,8 Healthcare providers should always practice prudent antibiotic use and consider local antibiotic resistance patterns to minimise the risk of antibiotic resistance development.9 References available on request

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(Mis)understanding the micro: Pseudomonas aeruginosa Glenn Valoppi, Slade Pharmacy Richmond Pseudomonas aeruginosa is an aerobic Gram-negative bacterium that wields a range of resistance mechanisms against antibiotics. It can also switch-on usually dormant defences, or acquire new protections - including enzymes that break down beta-lactam antibiotics, and efflux pumps (channels in the bacteria wall that pump the antibiotic out of the bacterial cell) - meaning the antibiotics can no longer work.

This means that there is only a small number of antibiotics that we can use against Pseudomonas aeruginosa, as listed in Figure 1, and resistance to these can develop. We are also beginning to learn about how best to dose and administer these antibiotics to make sure they are better able to overcome the bacteria’s defences. This typically means using larger or more frequent doses (such as piperacillin-tazobactam every 6 hours, instead of every 8 hours), or extending the duration of the infusion to keep drug levels maintained for a larger part of the time - to help prevent the bacteria resisting treatment by that antibiotic.

Figure 1: Typical Anti-Pseudomonal antibiotics1,2 Class

Examples on Australian market

Notes

Penicillins

Piperacillin-Tazobactam

Amoxicillin +/- Clavulanic Acid does not have anti-Pseudomonal activity.

Ticarcillin-Clavulanate no longer marketed in Australia.

Cephalosporins

Ceftazidime Cefepime

Cefazolin and ceftriaxone do not have anti-Pseudomonal activity.

Ceftolozane-Tazobactam* Ceftazidime-Avibactam* Cefiderocol* Carbapenems

Meropenem Imipenem-Cilastatin*

Ertapenem does not have antiPseudomonal activity.

Aminoglycosides

Tobramycin Amikacin* (Gentamicin)*

(Gentamicin may no longer be reported as having reliable antiPseudomonal activity)3,4,5 Aminoglycosides more often used as part of combination therapy, other than for lower UTIs.

Fluoroquinolones Ciprofloxacin (Norfloxacin – cystitis only) Others

Moxifloxacin does not have reliable anti-Pseudomonal activity, ciprofloxacin preferred.

Colistin*, Aztreonam* *Typically reserved antibiotics, usually restricted from publication in susceptibility reports

Microbiology reporting resistance Antimicrobial susceptibility reporting by pathology laboratories typically employ selective reporting to guide clinicians to the preferred drug therapy. This includes restricting testing for and/or hiding reporting of susceptibility to reserved therapies (such as cefiderocol) unless resistance to first-line options is reported. These reports may also include drugs which are not expected to have anti-Pseudomonal activity (always reported as resistant), to ensure clinicians understand to not expect therapeutic efficacy from these options.

Learning Point: While this report on a urine culture (Figure 2) might include multiple (R)-resistant tags, this is not a ‘multi-resistant Pseudomonas’ isolate. It retains expected susceptibility to the anti-Pseudomonal antimicrobials, and can be treated with typical guideline recommended therapy.

There is no need to assume a need to start meropenem, and/or employ infection control isolation precautions.

A susceptibility report for a more resistant Pseudomonas isolate is below (Figure 3). Given the evidence of resistance to at least one drug in at least three categories of typical anti-Pseudomonal drugs, this may be considered a multi-drug resistant Pseudomonas.6,7 Also note that information about aminoglycosides and reserved agents has not yet been made available – options for therapy may still exist, if antibiotic therapy is required.

Figure 2:

Figure 3:

Organism 1 Pseudomonas aeruginosa

10 - 100 X10*6 cfu/L

SENSITIVITIES: Organism 1 Ampicillin/Amoxycillin R Amoxycillin/Clavulanate R Cotrimoxazole R Ceftazidime S Cefazolin/Cephalexin R Gentamicin S Meropenem S Nitrofurantoin R Norfloxacin S Piperacillin/tazobactam S Tobramycin S Trimethoprim R

SPECIMEN:

ULCER SWAB RIGHT LEG

GRAM STAIN: Leucocytes: + Epi. cells: Nil

G+ve cocci: G-ve cocci:

Nil Nil

G+ve bacilli: G-ve bacilli:

Nil +

CULTURE: Organism 1 Pseudomonas aeruginosa

Moderate growth

SENSITIVITIES: Organism 1 Ampicillin/Amoxycillin R Amoxycillin/Clavulanate R Ciprofloxacin R Cotrimoxazole R Ceftazidime R Ceftriaxone R Cefazolin/Cephalexin R Meropenem R Piperacillin/tazobactam

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Microbiology reporting susceptibility (including Susceptible - Increased Exposure) For the laboratory to label an isolate as susceptible to a drug, there is an expectation that a suitable dose and frequency are being administered to reach, and remain at, the site of infection at concentrations that align with the antimicrobial activity.

This explains why some drugs which achieve high urinary concentrations may be reported as suitable for urine samples, but not for the same pathogen found in blood cultures or central nervous system infections.

Organisms may also be reported as susceptible contingent on increased exposure being achieved. This may be via higher doses, more frequent doses, or extended infusions, to keep concentrations above the minimum inhibitory concentration (MIC) for a longer

duration. The MIC is a measure of antibiotic concentration required to treat the bacterial infection, and increases as the ability of the organism to resist antibiotics grows. If the dose required to achieve antimicrobial activity is not safely achievable, or well tolerated in humans, the isolate is reported as resistant.

For Pseudomonas, and other organisms, there have been recent updates to MIC levels above which susceptibility cannot be expected at standard doses.3,4 Increased Exposure Dosing (IED) is required to deliver effective anti-Pseudomonal activity, particularly for non-urinary infections. Renal excretion of many antibiotics may achieve increased exposure in urine with standard doses. Refer to Figure 4 for dosing examples.

Figure 4: Increased Exposure dosing examples for Pseudomonas3,4 Drug

‘Standard’ Dosing used for infections not caused by Pseudomonas or other more resistant bacteria

Increased Exposure Dosing3,4 recommended for Pseudomonas or other more resistant bacteria

Adjustment needed for extremes of weight, and altered renal clearance Piperacillin-Tazobactam

4g/0.5g IV 8 hourly (30-minute infusions)

4g/0.5g IV 6 hourly (+/- administered as 3-hour infusion) or 4g/0.5g IV 8 hourly (administered as 4-hour infusion)

Cefepime

1g IV 8 hourly, or 2g IV 12 hourly

2g IV 8 hourly (+/- administered as 4-hour infusion) [especially if EUCAST reference used by local pathology laboratory]

Ceftazidime

1-2g IV 8 hourly

2g IV 8 hourly (or 1g IV 4-6 hourly)

Ciprofloxacin

500mg PO 12 hourly, or 400mg IV 12 hourly

750mg PO 12 hourly, or 400mg IV 8 hourly

Meropenem*

1g IV 8 hourly (30-minute infusions)

2g IV 8 hourly (+/- administered as 3-hour infusion)

*standard dosing remains appropriate for a Pseudomonas reported as susceptible without IED3,4

Drug selection When prescribing an empiric antibiotic with expectation of activity against possible Pseudomonas, an appropriate dose should be chosen. If activity against possible Pseudomonas or other more resistant bacteria is not required, then consider whether a narrower spectrum agent could be considered instead, e.g. amoxicillinclavulanate in settings such as abdominal infections or diabetic foot infections.

Gentamicin may no longer be reported by laboratories as having antiPseudomonal activity. Reported susceptibility to tobramycin should not be used to infer susceptibility to gentamicin and amikacin.

Tobramycin may be considered the preferred aminoglycoside, and should be used at recommended doses, with therapeutic drug monitoring to optimise delivery of optimal exposure.

Summary Pseudomonas aeruginosa can cause difficult to treat bacterial infections. There are a limited number of anti-Pseudomonal antibiotic options available on the Australian market. The bacteria have the ability to evade therapy using a range of innate and flexible methods. Recently updated microbiology references may result in different susceptibility reporting than we have been familiar. Successful outcomes are more likely with optimal dosing strategies, where the dose selected and the method of administration can make a difference and should be followed when recommended by the treating clinician.

References available on request.

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New Drug Brief: Faropenem Anthony Fico, Epic Pharmacy Murdoch Faropenem is an orally active penem antibiotic (subclass of β-lactams) structurally similar to carbapenems such as meropenem and ertapenem (See Figure 1).1,2 Backed by limited clinical evidence, faropenem is approved for use in Japan and India2 and may be appropriate to treat a range of community infections. It also provides an option for stepdown oral therapy for hospital infections.1 Faropenem has a broad spectrum of activity including many Gram-positive and Gramnegative aerobes, and anaerobes. It also has activity against β-lactamase producing organisms which are resistant to penicillins, making it a potential solution to combat the rise of resistant pathogens,1,3 however it is inactive against methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa.

S

S

N

N

Penam (penicillins)

Cephem (cephalosporins)

Faropenem is indicated for the treatment of various infections as an alternative to standard therapy when there is a concern with resistant pathogens, including:1,2,4 ¬

Upper and lower respiratory infections

¬

Uncomplicated skin infections

¬

Gynaecological infections

¬

Urinary tract infections

S N Penem

Dosing and administration: Available in tablet and liquid form, faropenem is generally well tolerated with the most frequently reported adverse events being gastrointestinal upset and rash. Dosing frequency is two or three times daily with general dosing guidelines from 200mg twice daily, up to 300mg three times a day.2,3,4 With excretion and metabolism via the kidneys, dose reduction is required in patients with renal impairment and caution is advised in combination with nephrotoxic drugs.4

N Carbapenem

Faropenem should not be used in individuals with a known hypersensitivity to penem or other β-lactam antibiotics such as penicillins or cephalosporins.

Penem Class

Although currently unregistered in Australia (available commercially in Japan and India2) faropenem can be accessed through the Therapeutic Goods Administration Special Access Scheme. Please consult your pharmacy department for more information.

Fig. 1 - General structures of penems and carbapenems as they relate to penams (penicillins) and cephems (cephalosporins).1

References available on request.

If you have any queries regarding Circuit content and authors please contact your pharmacy manager. Every effort has been made to ensure this newsletter is free from error or omission. sladepharmacy.com.au

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