IMHO kefir have to be among the best and more complete probiotic available.The culture can be grow in cow milk because it will consume the lactose which it is not digested by dogs.
Kefir grains are easy to reproduce and a culture can start with 2 tsp of grains in milk.
You will see the large type of yeasts in the grains but do not worry, they do not known to produce allergies.
Here are some of the ingredients in kefir grains (excuse me the long post)
LACTOBACILLI
Lactobacillus acidophilus
Lb. brevis [Possibly now Lb. kefiri]
Lb. casei subsp. casei
Lb. casei subsp. rhamnosus
Lb. paracasei subsp. paracasei
Lb. fermentum
Lb. cellobiosus
Lb. delbrueckii subsp. bulgaricus
Lb. delbrueckii subsp. lactis
Lb. fructivorans
Lb. helveticus subsp. lactis
Lb. hilgardii
Lb. helveticus
Lb. kefiri
Lb. kefiranofaciens subsp. kefirgranum
Lb. kefiranofaciens subsp. kefiranofaciens
Lb. parakefiri
Lb. plantarum
STREPTOCOCCI/LACTOCOCCI
Streptococcus thermophilus
St. paracitrovorus^
Lactococcus lactis subsp. lactis
Lc. lactis subsp. lactis biovar. diacetylactis
Lc. lactis subsp. cremoris
Enterococcus durans
Leuconostoc mesenteroides subsp. cremoris
Leuc. mesenteroides subsp. mesenteroides
Leuc. dextranicum
YEASTS
Dekkera anomala t/ Brettanomyces anomalus a
Kluyveromyces marxianus t/ Candida kefyr a#
Pichia fermentans t/ C. firmetaria a
Yarrowia lipolytica t/ C. lipolytica a
Debaryomyces hansenii t/ C. famata a#
Deb. [schwanniomyces] occidentalis
Issatchenkia orientalis t/ C. krusei a
Galactomyces geotrichum t/ Geotrichum candidum a
C. friedrichii
C. rancens
C. tenuis
C. humilis
C. inconspicua
C. maris
Cryptococcus humicolus
Kluyveromyces lactis var. lactis #
Kluyv. bulgaricus
Kluyv. lodderae
Saccharomyces cerevisiae #
Sacc. subsp. torulopsis holmii
Sacc. pastorianus
Sacc. humaticus
Sacc. unisporus
Sacc. exiguus
Sacc. turicensis sp. nov
Torulaspora delbrueckii t
* Zygosaccharomyces rouxii
ACETOBACTER
Acetobacter aceti
Acetobacter rasens