Selection of antimicrobial agents

Selection of antibiotics for masitis

Successfully treating mastitis starts with an appropriate drug choice. Although not perfect, microbiological culturing and antibiotic susceptibility testing are still the most feasible options worldwide to determine which antimicrobial has the best chance for success.

Which antibiotics should be tested for susceptibility?

The number of antimicrobial agents that can be applied in a routine antibiogram is limited. To make testing relevant and practical, it makes sense to include those antibiotics for which commercial preparations are available on the market. In most antibiotic groups, representative molecules can be selected with the same spectrum of activity and full cross-resistance. This approach can deliver reliable and consistent results. In some antibiotic classes, such as the aminoglycosides, this is not possible. All molecules of this group show incomplete cross-resistance. However, in general, testing each antimicrobial available on the market is not necessary.

In addition to choosing representatives of antibiotic classes, discs that test for specific types of antimicrobial resistance should also be added. An example of this is cefoxitin for MRSA detection in Staphylococcus spp. and cefotaxime for ESBL detection in Enterobacteriaceae.

Antimicrobial classes

Only those antimicrobial classes that are relevant in the treatment of mastitis are mentioned in this article.


  • Narrow-spectrum, β-lactamase-susceptible penicillins are primarily active against non-β-lactamase-producing, Gram-positive bacteria such as streptococci and staphylococci. They also act against some fastidious bacteria such as Pasteurella. This group consists of: benzathine penicillin, benzyl penicillin (pen G), procaine penicillin and penethamate. Susceptibility is tested with a penicillin (1 unit) disc.
  • Narrow-spectrum, β-lactamase- resistant penicillins are also active against Gram-positive bacteria such as streptococci and staphylococci, including the β-lactamase-producing Staphylococcus spp. This group consists of cloxacillin, dicloxacillin, flucloxacillin, methicillin, nafcillin and oxacillin. Susceptibility of staphylococci is tested with a cefoxitin disc (30 μg), that of streptococci with an oxacillin disc (1μg).
  • Moderate-spectrum penicillins (ampicillin and amoxicillin) are active against streptococci, non-β-lactamase-producing staphylococci as well as against Gram-negative species, including some members of Enterobacteriaceae such as E. coli, Citrobacter and Enterobacter spp. This group consists of ampicillin and amoxicillin. The susceptibility of Enterobacteriaceae is tested with an ampicillin disc (10 μg), that of staphylococci and streptococci with a penicillin disc (1 unit).


β-lactamase inhibitor combinations

These antimicrobial agents are combinations that include a penicillin-class antimicrobial agent and a second agent that has minimal antimicrobial activity, but functions as an inhibitor of most β-lactamases. In veterinary medicine, only clavulanic acid is currently used in some countries in combination therapies against mastitis.

The susceptibility of staphylococci to amoxicillin with clavulanic acid is tested with a cefoxitin disc (30 μg), and the susceptibility of Enterobacteriaceae with an amoxicillin/ clavulanic acid disc (20/10 μg). Streptococci are not able to produce β-lactamase, therefore testing is not applicable.


The various representatives of cephalosporins can have a somewhat different activity against Gram-positive and Gram-negative bacteria. These agents are often referred to as 1st, 2nd, 3rd and 4th generation cephalosporins, based on the extent of their activity against the more antimicrobial-resistant, Gram-negative bacteria.

  • First (and second) generation cephalosporins are active against Gram-positive streptococci and staphylococci, including the β-lactamase-producing staphylococci, but not against methicillin-( oxacillin) resistant staphylococcus (e.g. MRSA or methicillin-resistant Staphylococcus aureus). They are active against wild types of Enterobacteriaceae.This group includes cefalexin, cefalonium, cefalotin and cefapirin. Cefalexin (30 μg) discs are used to test the susceptibility of Enterobacteriaceae, cefoxitin discs (30 μg) are used for staphylococci and penicillin discs (1 unit) are used for streptococci.
  • Third and fourth generation cephalosporins have a broad spectrum of activity both for Gram-positive and Gram-negative bacteria. They are generally resistant to a broad spectrum of β-lactamases, which makes them especially valuable for the treatment of Gram-negative infections. However, resistance occurs and their use should be carefully considered*. This group consists of ceftiofur, cefoperazone and cefquinome. The susceptibility of Enterobacteriaceae to these antimicrobials can be assessed with a cefotaxime disc (5 μg); however, the use of a disc with the specific therapeutic antimicrobial could result in different outcomes. The susceptibility of staphylococci is tested with a cefoxitin disc (30 μg) and streptococcal susceptibility is assessed with a penicillin disc (1 unit).

 *Note: According to the WHO, 3rd and 4th generation cephalosporins are considered to be very critical antibiotics for human medicine and their use in veterinary medicine should therefore be carefully considered.


Aminoglycosides are bactericidal antimicrobials with both Gram-positive and Gram-negative activity. Generally, this includes Staphylococcus spp. (including β-lactamase- producing species) and EnterobacteriaceaeStreptocococcus spp. are naturally resistant to all molecules of this class. This group of chemically-related drugs includes, among others, kanamycin, streptomycin, neomycin, gentamicin and spectinomycin.

Framycetin is one of the compounds of neomycin; in fact, it is its strongest-acting compound. Aminoglycosides are affected in variable ways by bacterial inactivating enzymes, which results in differences in their spectrum of activity. For this reason, each aminoglycoside should be tested separately for a reliable evaluation of its efficacy.

Susceptibility is tested with a kanamycin disc (30 μg), a neomycin disc (10 μg), a gentamicin disc (10 μg) and an amikacin disc (30 μg).



Macrolides are bacteriostatic agents that inhibit protein synthesis. This group includes erythromycin, gamithromycin, tilmicosin, tulathromycin and tylosin. They are active against staphylococci and streptococci although resistance occurs. The mastitis- causing, Gram-negative Enterobacteriaceae and Pseudomonas spp. are naturally resistant.

Susceptibility is tested with an erythromycin disc (15 μg).


Lincosamides are moderate-spectrum antimicrobial drugs. These molecules inhibit protein synthesis and include lincomycin, clindamycin and pirlimycin. They are efficacious against most Gram-positive bacteria (staphylococci, streptococci, but not enterococci) and against Gram-positive and -negative anaerobe bacteria. These drugs are not active against most aerobic or facultative aerobic Gram-negative bacteria such as Enterobacteriaceae.

Susceptibility is tested with a clindamycin disc (2 μg) and inducible resistance is tested with a side-by-side test next to an erythromycin disc (15 μg) (see Special cases).

Trimethoprim and Sulfonamides

Sulfonamides are broad-spectrum antimicrobial agents usually combined with trimethoprim because of the synergy between both molecules. This combination affects different steps of the production of folic acid and thus that of purines, which are equired for DNA synthesis. Their spectrum includes Gram-positive and Gram-negative bacteria and some protozoa. However, they are not active against anaerobe bacteria and at locations with tissue debris. Trimethoprim-sulfonamide combinations (TMPS) are active against staphylococci, streptococci and Enterobacteriaceae.

Susceptibility is tested with a Trimethoprim- sulfamethoxazole disc (1.25-23.75 μg = 25 μg).

*Note: According to the WHO, macrolides are considered to be very critical antibiotics for human medicine and their use in veterinary medicine should therefore be carefully considered.



Tetracyclines are bacteriostatic antimicrobials, which have a broad spectrum of activity against Gram-positive and Gram-negative bacteria. This group includes tetracycline, oxytetracycline, chlortetracycline and doxycycline and susceptibility is tested with a tetracycline disc (30 μg).


This group includes the older quinolones (e.g. nalidixic acid) and the newer fluoroquinolones, such as danofloxacin, enrofloxacin, marbofloxacin. All quinolones act by blocking DNA replication, and some quinolones also act on non-dividing bacteria. Especially the newer fluoroquinolones have a broad spectrum of activity against aerobic Gram-negatives and Gram-positives. They have a good activity against staphylococci and Enterobacteriaceae.

Susceptibility is tested with an enrofloxacin disc (5 μg) or a ciprofloxacin disc (5 μg).


Several antimicrobial combinations have been approved for intramammary use. Especially for the combinations where a synergy has been proven against mastitis pathogens, the separate testing of molecules will underestimate sensitivity. Testing of the combination at ratios that are likely to be reached in the udder is therefore advised.

Cefalexin/kanamycin is an example of a combination with established synergy3 against Staphylococcus aureus, Streptococcus uberis, Streptococcus dysgalactiae and Escherichia coli. Susceptibility is tested with a cephalexin-kanamycin disc (C 15μg/ K 30 μg).

 *Note: According to the WHO, fluoroquinolones are considered to be very critical antibiotics for human medicine and their use in veterinary medicine should therefore be carefully considered.

 3 Ganiere et al. 2009. Synergistic interactions between cefalexin and kanamycin in Mueller-Hinton broth medium and in milk. Journal of Applied Microbiology, 107: 117-125
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