NEW YORK — Standard & Poor’s Ratings Services has lowered its credit ratings for many of the world’s largest financial institutions, including the biggest banks in the U.S.

Bank of America Corp. and its main subsidiaries are among the institutions whose ratings fell at least one notch Tuesday, along with Citigroup, Goldman Sachs Group, JPMorgan Chase, Morgan Stanley and Wells Fargo.

S&P said the changes in 37 financial companies’ ratings reflect the firm’s new criteria for banks, and they incorporate shifts in the industry and the role of governments and central banks worldwide. The agency did not release its evaluation of each company but said it planned to discuss the changes during a conference call earlier today.

Bank of America’s issuer credit rating was cut to “A” from “A+,” while its Countrywide Financial Corp. and Merrill Lynch units and a series of related subsidiaries were cut to “A-” from “A.”

Ratings downgrades are never seen as positive, but this round may be particularly damaging for Bank of America.

Concern already was growing about whether B of A has enough capital to withstand another downturn in the U.S. economy or further trouble in Europe, and the bank’s stock fell to a two-year low before the ratings announcement.

The Charlotte, N.C.-based bank said in a recent regulatory filing that downgrades from S&P or Fitch Ratings, which also is reevaluating its ratings, “could likely have a material adverse effect on our liquidity” and cut off its access to credit markets.

It typically costs companies more to borrow when their credit ratings are cut, the same way a decline in a person’s credit scores drives up the interest rates that banks and credit cards offer him.

Downgrades could hurt parts of the bank’s businesses where creditworthiness is critical, Bank of America said in a filing Nov. 3 with the Securities and Exchange Commission.

A downgrade also could trigger provisions in derivative contracts that require B of A to put up more collateral, and it could terminate the contracts, resulting in losses and hurting the bank’s liquidity.

The bank posted a $6.2 billion profit for the third quarter, mostly the result of accounting gains and the sale of a stake in a Chinese bank, but it was still moving toward a loss for the year as of Sept. 30.