Short communication: Comparative performance of 3 on-farm culture systems for detection of mastitis pathogens interpreted by trained and untrained observers

Abstract

On-farm culture (OFC) systems facilitate pathogenbased mastitis management and can facilitate antimicrobial stewardship on dairy farms. Interpretation of the results, however, may present a challenge for those with limited microbiology experience. Here, we compared results of 3 OFC systems interpreted by trained and untrained observers against results of a standard laboratory reference method (aerobic culture and mass spectrometry). Milk samples (280 quarter and 60 composite) were selected from submissions for routine diagnostic testing to Quality Milk Production Services (Cornell University, Ithaca, NY) between August 2017 and January 2018. Samples were cultured simultaneously using the standard laboratory reference method and 3 commercially available OFC systems that varied in detail of pathogen identification (provided in parentheses) as follows: (1) Minnesota Easy Culture System II Bi-plate (University of Minnesota Laboratory for Udder Health, St. Paul; gram-positive, gram-negative), (2) Minnesota Easy Culture System II Tri-plate (gram-positive, gram-negative, some genus level), and (3) FERA Diagnostics and Biologicals AccuMast plate (Ithaca, NY; genus level, some species level). After 18 to 24 h of incubation, OFC plates were interpreted by 1 trained observer (>10 yr of experience in milk microbiology) and 6 untrained observers with no previous milk microbiology training, using only the manufacturers' instructions for guidance. Strength of agreement (κ) between observer groups and the reference method was determined for the available outcomes of each system. Interpreted by the trained observer, agreement was moderate for identifying gram-positive organisms (Bi-plate, κ = 0.56) and substantial for Streptococcus spp. (Tri-plate, κ = 0.64, AccuMast κ = 0.61). Interpretation by untrained observers resulted in fair agreement (κ = 0.29-0.37) for these organisms. Moderate agreement (κ = 0.43-0.59) was found across all 3 OFC for the identification of gram-negative organisms (Bi-plate), non-aureus staphylococci (Tri-plate and AccuMast), Lactococcus spp., and Enterococcus spp. (AccuMast) when interpreted by the trained observer, and fair to moderate agreement was found (κ = 0.31-0.53) among untrained observers. Across all 3 OFC, agreement was almost perfect (κ = 0.80-0.89) for Staphylococcus aureus for the trained observer, and moderate to substantial (κ = 0.56-0.61) for untrained observers. We concluded that all 3 OFC appeared suitable to support pathogen-based mastitis management when operated by trained observers. Training beyond the instruction manual is a prerequisite to make OFC systems useful for pathogen-based mastitis management.

Comparison of Immediate Blanket Treatment versus a Delayed Pathogen-Based Treatment Protocol for Clinical Mastitis Using an On-Farm Culture Test at a Commercial German Dairy Farm

“…The accuracy of the Accumast ® system in identifying E. coli and no growth was 96.8% and 85.2%, respectively [10]. These results might differ when an on-farm culture system is used by farm personnel rather than trained laboratory technicians, as shown recently [15]. In that study, experience in milk microbiology substantially improved interpretation of on-farm culture results, indicating that the observers' experience is crucial to facilitate appropriate management decisions when adopting a selective treatment protocol.…”

Udder Health Monitoring for Prevention of Bovine Mastitis and Improvement of Milk Quality

“…However, although these on-farm culturing systems have become commercially available at reasonable prices, some of the studies have pointed out that the commercial on-farm culturing systems differed significantly in their ability to classify bacterial colonies by genus and species [ 153 ] and training beyond the instruction manual is required for untrained observers to make this type of systems effective for pathogen-based mastitis control [ 154 ].…”

A framework for evaluation of on‐farm mastitis diagnostics in Australia

“…Although visually appealing, the interpretation of chromogenic assays by microbiologically untrained users may lead to limited sensitivity and specificity. 18 With rare exceptions, 16 most peer-reviewed publications of 'on-farm tests' describe the evaluation of such assays in the laboratory, 15,19,20 or their use on-farm by trained research personnel. 21 The accuracy of POC tests for mastitis may be overestimated in such studies when compared to actual on-farm use, both through interpretation of results by users that are more qualified than the intended end-user and through set-up by trained personnel, which reduces the risk of sample contamination.…”

“…Sipka and colleagues, however, caution against the use of on-farm diagnostics to inform such definitive decisions because the accuracy of diagnoses by untrained users is limited. 18 In addition, the prognosis for cure is highly dependent on cow factors and penicillin susceptibility of the isolate so a preliminary diagnosis of S. aureus may warrant additional testing prior to culling decisions. 3 Bi-plate Like the use of Petrifilm AC and CC, bi-plate systems allow for the growth of gram-positive and gram-negative organisms and for the detection of culture-negative samples.…”