I've searched for some time now, but I can't find a definitive answer. The closest I have gotten is "knowledge about peptidoglycan structures dates back to the 1970s–80s" from this paper.
The discovery of peptidoglycan, as present in the bacterium cell wall took many years.
“The terms mucopeptide, glycopeptide, or murein, used by some authors, are all synonymous with peptidoglycan.” [Ghuysen, J.M. (1968) Bacteriol. Rev. 32:425-64] Among those, Weidel and Pelzer previously coined the term murein [Weidel, W. and Pelzer, H. (1964) Adv. Enzymol. Rel. Areas Mol. Biol. 26:193].
Regarding the organized, higher structure of the peptidoglycan as found in the Gram-negative cell envelope, Salton [Salton, M.R.J. (1994) New Compr. Biochem. 27:1-22] mentioned the following:
“The exact nature of comparable ‘wall’ fractions obtained from Gram-negative bacteria remained a matter of uncertainty until the advent of electron microscopy of ultra-thin sections of bacterial cells as demonstrated by Ryter and Kellenberger [Ryter, A. and Kellenberger, E. (1958) J. Biophys. Biochem. Cytol. 4:671-678.]. The response of cells to the Gram stain procedure and the early chemical analysis of ‘cell wall’ preparations clearly pointed to some basic differences in the properties of the envelopes of Gram-positive and Gram-negative bacteria and the greater chemical complexity of the isolated fractions from the latter organisms [Salton, M.R.J. (1953) Biochim. Biophys. Acta 10:512-523.]. The structural complexity of the Gram-negative cell envelope was clearly resolved in the thin sections of Ryter and Kellenberger [Ryter, A. and Kellenberger, E. (1958) J. Biophys. Biochem. Cytol. 4:671-678.] and established the presence of an outer membrane as a characteristic feature of the cell envelope. The mysteries of the structure of the Gram-negative cell envelope were finally solved with the demonstration of the thin layer (peptidoglycan) between the outer and inner (cytoplasmic) membranes by use of lanthanum-stained thin sections by Murray et al. [Murray, R.G.E. et al. (1965) Can. J. Microbiol. 11:547-560.], and the isolation and demonstration of the thin murein sacculus of E. coli by Weidel et al. [Weidel, W. et al. (1960) J. Gen. Microbiol. 22:158-166.]. Thus, the complex cell envelopes of Gram-negative bacteria were resolved as consisting of an outer membrane anchored to the underlying rigid, thin, peptidoglycan layer or sacculus.”