modify strip labels with given text using lattice package

Dear all,
I am drawing a barchart plot with lattice and the resulting strips are
taking the value of the variable being compared (in this example
"assay"). However I would like to write myself the value to place into
the strips, let's say I want to call the variables as "molecular test"
and "serological test" the values "a" and "b" respectively within
"assay". I have tried different approaches taken from the web but
nothing worked.
Would you have any tip?
Best regards
Luigi

test <- rep(c("Adenovirus", "Rotavirus", "Norovirus", "Rotarix",
"Sapovirus"), 2)
res <- c(0, 1, 0, 1,0, 1,0, 1,0, 1, 0, 1, 0, 1,0, 1,0, 1,0, 1)
count <- rnorm(20)
assay <- c(rep("a", 10), rep("b", 10))

df <- data.frame(test, res, count, assay, stringsAsFactors = FALSE)

library(lattice)
barchart(
    test ~ count|assay,
    df,
    groups = res,
    stack = TRUE,
    main = "Comparison of test results",
    xlab = "Count",
    col = c("yellow", "blue"),
    par.settings = list(
        strip.background = list(col="light grey"),
        superpose.polygon=list(col= c("yellow", "blue"))
    ),
    scales = list(
        alternating = FALSE
    ),
    key = list(
        space="top",
        columns=2,
        text=list(c("Negative", "Positive"), col="black"),
        rectangles=list(col=c("yellow", "blue"))
    )
)