Corynebacterium bovis (Cb), the etiologic agent of Corynebacterium-associated hyperkeratosis (CAH) in nude mice, may impact research outcomes. Little is known about the differences in the course and severity of CAH in different outbred athymic nude mice stocks. Three genetic stocks (designated A, B, and C), 1 of which was obtained from 2 geographically separate colonies with distinct microbiota (A1 and A2), were inoculated topically with 1 × 108 cfu of a pathogenic Cb field isolate (no. 7894; n = 6/stock) or sterile media (n = 2/stock; controls). Clinical signs were assessed daily and scored 0 to 5 based on lesion severity. Mice were euthanized at 14 (A1, A2, B, and C) or 28 (B) days postinoculation (dpi), macroscopic changes were documented, and 6 skin samples per mouse were obtained and histologically scored 0 to 4 based on the presence and severity of hyperkeratosis, acanthosis, inflammation, and bacterial colonies. No stock A1 or control mice developed clinical disease; 1 of 6 stock B mice developed mild CAH (mean peak clinical score [MPCS]: 0.33) at 14 dpi (14-d group); 2 of 6 stock B (28-d group) developed mild CAH at 15 dpi (MPCS: 0.33); and all stock C and A2 mice developed significant clinical signs at 5 dpi (MPCS: 2.5 and 3, respectively), which resolved by 11 dpi. Despite differences in clinical presentation, all Cb-infected mice had hyperkeratosis and/or acanthosis with associated bacterial colonies. Stocks A1 and B, which had minimal or no clinical signs, were colonized with Corynebacterium amycolatum (Ca). In contrast, stocks C and A2 were not colonized with Ca, raising the possibility that Ca and/or other components of the skin microbiota may mitigate clinical signs but not necessarily all histopathologic changes associated with infection. These findings suggest host genetics and/or the skin microbiota can markedly influence the presentation of CAH in nude mice.
Photographs demonstrating the clinical scoring system used to assess Cb-associated skin disease in athymic nude mice at necropsy. Mice were scored 0 to 5 based on clinical signs representing mild, moderate, and severe disease. Reprinted with permission from Mendoza et al.13
Photomicrographs demonstrating the histologic scoring system used for assessing the severity of acanthosis and hyperkeratosis in athymic nude mice infected with Cb. Samples were semiquantitatively scored as normal (0), minimal (1), mild (2), moderate (3), or severe (4), based on the intensity of acanthosis and orthokeratotic hyperkeratosis. A0 to A4, acanthosis grade; H0 to H4, hyperkeratosis grade; black bracket represents the location and amount of acanthosis (A) or hyperkeratosis (H). Reprinted with permission from Mendoza et al.13
Clinical scores by stock and colony geographical site of origin. Scores reflect the mean ± SD for n = 6 mice/stock assessed daily. Stock A mice were obtained from 2 geographical sites with mice from A1 colonized with Ca while A2 were Ca-free. Stock B mice were evaluated for either 14 or 28 dpi. *, P ≤ 0.05 when comparing stock A2 and C to A1 and B (14- and 28-d study). **, P ≤ 0.05 when comparing stock C to stocks A1 and B (14- and 28-d study). Kruskal-Wallis and Dunn multiple comparisons tests.
Stocks with Ca as part of the normal skin microbiota had a statistically significant lower MPCS than those without Ca (*, P ≤ 0.05). Mann-Whitney U test. Scores from the 28 dpi group were excluded from the statistical analysis.
Graphical presentation of histopathology scores for 3 Cb-infected and uninfected control outbred nude stocks (A, B, and C) and geographical sites of origin (A1 and A2) evaluated 2 and/or 4 (B only) weeks postinoculation. Scores reflect the average distribution of the means (6 biopsies/mouse) of the 6 Cb-infected or 2 uninfected control mice/stock for each of 4 histopathology criteria assessed. Kruskal-Wallis and Dunn multiple comparisons tests were used to compare histopathology scores of mice for 14 d postinfection. The Mann-Whitney U test was used to compare histopathology scores of stock B mice at different time points and between Cb-infected and uninfected controls of each stock at each time point. *P ≤ 0.05.
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