- Open Access
Sensitivity of two methods to detect Mycoplasma agalactiae in goat milk
© Tatay-Dualde et al. 2015
- Received: 9 October 2014
- Accepted: 3 September 2015
- Published: 7 September 2015
Laboratory diagnostic techniques able to detect Mycoplasma agalactiae are essential in contagious agalactia in dairy goats. This study was designed: 1) to determine the detection limits of PCR and culture in goat milk samples, 2) to examine the effects of experimental conditions including the DNA extraction method, PCR technique and storage conditions (fresh versus frozen stored milk samples) on these methods and 3), to establish agreement between PCR and culture techniques using milk samples from goats with mastitis in commercial dairy herds. The study was conducted both on artificially inoculated and field samples.
Our findings indicate that culture is able to detect M. agalactiae in goat milk at lower concentrations than PCR. Qualitative detection of M.agalactiae by culture and PCR was not affected by sample freezing, though the DNA extraction method used significantly affected the results of the different PCR protocols. When clinical samples were used, both techniques showed good agreement.
The results from this study indicate that both culture and PCR are able to detect M. agalactiae in clinical goat mastitis samples. However, in bulk tank milk samples with presumably lower M. agalactiae concentrations, culture is recommended within the first 24 h of sample collection due to its lower limit of detection. To improve the diagnostic sensitivity of PCR in milk samples, there is a need to increase the efficiency of extracting DNA from milk samples using protocols including a previous step of enzymatic digestion.
- Contagious agalactia
- Mycoplasma agalactiae
Contagious agalactia (CA) affects small ruminants and may cause a triad of clinical symptoms comprising keratoconjunctivitis, polyarthritis and mastitis. In addition, the syndrome may also produce respiratory and reproductive symptoms in goats. The disease is caused by several bacteria of the genus Mycoplasma although M. agalactiae is its main causative agent .
To determine the epidemiological situation of goat herds in endemic areas of contagious agalactia (CA), systematic diagnostic studies carried out on milk samples are strongly recommended . There are two kinds of diagnostic methods used to detect infected herds: indirect serological detection, exclusively when animals are not vaccinated, and direct evidence of M. agalactiae in milk by means of culture and/or PCR . In these studies, bacteriological techniques need to be conducted on milk samples in order to specifically detect the presence of CA causative agents. Therefore, particular media such as Eaton, Hayflick, pH or SP4II are recommended . PCR protocols designed for the identification of the mycoplasma species involved in caprine CA offer more rapid results and improved sensitivity [4–6]. However, DNA extraction and purification methods have also been considered to play a major role in the obteining of PCR results . In this sense, Tola et al.  developed a method to extract DNA directly from milk samples, and nowadays some commercial kits are also available for this purpose.
The aim of this study was to determine the lower limits of detection of culture and PCR techniques for M. agalactiae in experimentally inoculated goat milk samples. We also examined the effects of sample freezing and the DNA extraction method used on these techniques. In a second experiment, agreement between PCR and culture techniques was assessed using milk samples from mastitic goats from dairy herds of a CA endemic area.
For the experimental study, five goat milk samples were obtained: one bulk tank milk sample and four from randomly selected goats. All samples tested negative for M. agalactiae by conventional culture  and PCR .
PRC conditions for each protocol
Tola et al. (1996) 
94 °C, 5 min.; (94 °C, 1 min. 64 °C, 1 min.; 72 °C,1 min) x 35 cycles; 72 °C, 10 min.
Marenda et al. (2005) 
94 °C, 5 min.; (94 °C, 1 min.; 58 °C, 1 min.; 72 °C, 1 min.) x 30 cycles; 72 °C, 10 min.
De la Fe et al. (2012) 
94 °C, 2 min.; (94 °C, 30 sec.; 49 °C, 30 sec,; 72 °C, 30 sec.) x 30 cycles; 72 °C, 5 min.
For the field study, 255 clinical mastitis samples were obtained from 80 dairy goats herds reared in an endemic CA area. All samples collected were kept refrigerated at 4 °C and arrived at our laboratory within the following 24 h. Immediately after their arrival, these samples were cultured following the same protocol described in our experimental study. Then, 200 μl of each milk sample were processed with DNA extraction prior to PCR diagnosis [8, 5].
Agreement between PCR and culture techniques was determined using Win Episcope 2.0 software . Thursfield criteria  were used to interpret kappa values. The Chi squared test implemented in Epi Info 3.5.2 software  was used to assess the factors that affect the sensitivity of the techniques, which was calculated as the proportion of inoculated samples detected by the different methods used.
PCR results obtained according to the DNA extraction method and the Mycoplasma agalactiae concentration used
M. agalactiae concentration (CFU/ml)
Method of extraction
Tola et al. (1997) 
Se (%), 95 % CI
Se (%), 95 % CI
3.3, (−3.1 %, 9.8 %)
20, (5.7 %, 34.3 %)
6,7, (−2.3 %, 15.6 %)
26.7, (10.8 %, 42.5 %)
50, (32.1 %, 67.9 %)
73.3, (57.5 %, 89.2 %)
93,3, (84.4 %, 102.3 %)
33.8,a (37.8 %, 51.4 %)
44.6,b (27.3 %, 40.3 %)
Culture results according to the concentration of Mycoplasma agalactiae used to inoculate the goat milk samples
M. agalactiae concentration (CFU/ml)
In previous studies, it has been observed that M. agalactiae can be found in milk at concentrations of 106 up to 108 CFU/ml, . Moreover, Castro-Alonso et al.,  showed by experimental inoculation that this concentration could even reach values of 1010 to 1012 CFU/ml. Therefore, considering our limits of detection (above 106 with culture and PCR), both PCR and culture methods should be able to detect the presence of mycoplasmas from the initial days postinfection when the concentrations of M. agalactiae are above 106 CFU/ml, because both techniques show high values of sensitivity [15, 16]. However, when the concentration of mycoplasmas is unknown, as it happens with the bulk tank milk samples, where obviously these values are lower than in individual samples from infected animals, the use of PCR protocols for bulk tank samples with a limit of detection of 102–104 could be inadequate. Therefore, for the diagnosis of CA from bulk tank milk, samples should be previously cultivated and the DNA extraction should be carried out on the culture rather than on milk sample, in order to improve its sensitivity .
Of the 255 clinical samples examined, 58 were culture positive and 47 were PCR positive for M. agalactiae, while 41 samples were culture and PCR positive and 191 scored negative using both techniques. Our agreement study revealed a kappa coefficient of 0.725, indicating a good agreement between the two methods . This is probably due to the concentration range of M. agalactiae which is usually found in mastitic goat milk samples (106 to 1012 CFU/ml), that has been previously described [15, 16].
In conclusion, our findings show that both culture and PCR are able to detect M. agalactiae in clinical goat mastitis samples. However, in bulk tank milk samples with presumably lower M. agalactiae concentrations, culture within the first 24 h of sample collection is recommended. In order to improve the diagnostic sensitivity of PCR in milk samples, it is also necessary to increase the efficiency of the DNA extracting methods from milk samples, for example by using protocols with a previous enzymatic digestion so as to reduce the presence of inhibitors in our sample.
This study was conducted within the framework of Project AGL2013-44771-R financed by the Dirección General de Investigación y Gestión del Plan Nacional De I + D + i of the Spanish Ministerio de Economia y Competitividad.
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